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Suzuki GP100 bogging at Full Throttle

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Steve_W
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Joined: 14 May 2019
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PostPosted: 19:42 - 14 May 2019    Post subject: Suzuki GP100 bogging at Full Throttle Reply with quote

Evening everyone, new member here.

Having a bit of a mid life crisis ( Laughing ) so decided to buy an old 1984 Suzuki GP100 because I had one 30 years ago when I was 17!

I have a bit of an issue with it though and wondered if anyone has any ideas or is able to maybe back up what I am thinking could be the problem.

Bike ticks over fine, pulls fine up to half throttle and rides great in 30 - 40 mph limits when you don't need to give it more than half throttle, but when I open it up to full throttle it dies, until I move throttle back to half throttle and then it runs fine again.

Initially thought it was carb related, bike has been looked after by previous owner but has been stood around for some time, covered 2 miles in the last two years according to MOT history (so I guess just ridden to get MOT and back again!). So I booked it into local bike shop to get carb cleaned. After clean it was still doing it so bike shop then suggested it could be electrical. He did check the timing and has confirmed it's now spot on and the points are in a decent enough condition with no pitting

I have also checked the oil pump settings and adjusted according to Haynes Manual so I know that's correct.

So, since then I have replaced the following so far:-

Spark plug - goes without saying on a 2 stroke really I guess.
Condenser - there was a fair bit of arcing across the points before and now there is hardly any, as you'd expect I think.
Ignition coil and plug cap - after drilling out the seized screws! Laughing

So before I get to changing the stator plate (bought a second hand one from ebay as almost impossible to find new) as it's a bit of a ball ache and I've already soldered the new condenser onto the original stator, I decided to run it in the garage on the stand and get it to die, as before it's always been out on the road and I've not been able to check the plug til I've got back home.

it kicks over and starts beautifully now since I've replaced all of the above, comes off choke after a few seconds and idles just fine, revved to 7k with no issue with throttle below half open. But the minute I open the throttle wide open it dies and stops.

Took the plug out and it was drenched.

So, decided to take another look at the carb and the main jet size as that's what controls the throttle range I'm having problems with right? The size that's in there is 105. According to Haynes it should be 97.5 (or 105 for the "D" model).

I've ordered a 97.5 main jet and just waiting for it to arrive to see if it fixes my issue.

I suppose my questions are:-

does anyone on here know what the difference is between the "C, N, X and D" models of the GP100U and how I tell which one I have?

And do you think my thoughts are correct that it may be that the main jet is too big and it's flooding the engine at full throttle?

Sorry for the essay, but I've searched and searched the web for a while over this and thought I'd finally ask rather than just looking at other posts!
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Nobby the Bastard
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PostPosted: 20:25 - 14 May 2019    Post subject: Reply with quote

I had one the same sort of age at around the same time.

I wouldn't waste my time with one now. Get a modern bike.
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Steve_W
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PostPosted: 20:43 - 14 May 2019    Post subject: Reply with quote

Nobby the Bastard wrote:
I had one the same sort of age at around the same time.

I wouldn't waste my time with one now. Get a modern bike.
Nah, you're missing the point of why I bought it Wink

I could have bought a bigger, more modern bike for less than I paid for the GP.

I just remembered my old W reg one with rose tinted glasses and forgot how crap they were Laughing

I wanted something "classic" that meant something to me, and can't really justify spending in excess of 6-7000 for a Mk1 Fiesta Supersport - so bought a GP100 instead.

I'm actually quite enjoying working my way through it trying to fix this issue. It'll be great once it's fixed. Until next week when something else breaks no doubt Thumbs Up
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Nobby the Bastard
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PostPosted: 20:45 - 14 May 2019    Post subject: Reply with quote

Okwy, does it have the original airbox, fikter and exhaust type?
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Steve_W
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PostPosted: 20:49 - 14 May 2019    Post subject: Reply with quote

Yeah, original airbox and exhaust.

I've cleaned the air filter too, not that it was particularly dirty anyway but was a quick easy thing to do to rule out.
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Nobby the Bastard
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PostPosted: 21:02 - 14 May 2019    Post subject: Reply with quote

Do a plug chop on wide open throttle. My bet is the main jet is blocked.
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Steve_W
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PostPosted: 21:08 - 14 May 2019    Post subject: Reply with quote

Took the plug out at the weekend when I got it to die when sat in the garage rather than when I was out on the road, colour didn't look too bad but it was drenched in fuel.

I've assumed that means it's far too rich and is flooding at full throttle?

The jet looks clear, it's not blocked and the carb was sonically cleaned by a bike shop a few weeks ago, but as menationed in my first post it's a 105 main jet and according to Haynes most GP100s should have a 97.5 main jet.
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Nobby the Bastard
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PostPosted: 21:15 - 14 May 2019    Post subject: Reply with quote

Whip the air filter out and try. Tge increase in air should compensate to a degree if its too rich.
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Steve_W
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PostPosted: 21:19 - 14 May 2019    Post subject: Reply with quote

Yeah good point, I've got a 97.5 jet on order anyway but could try that at the weekend first (I don't get much in the way of spare time in the week unfortunately Crying or Very sad )
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jaffa90
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PostPosted: 00:56 - 15 May 2019    Post subject: Reply with quote

"""So, decided to take another look at the carb and the main jet size as that's what controls the throttle range I'm having problems with right?""""???????????


Well there is the tapered needle position in the main jet, also the fuel level height a bit to high. Or maybe the exhaust system is partly blocked and limiting the out flow??????
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Steve_W
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PostPosted: 09:29 - 15 May 2019    Post subject: Reply with quote

jaffa90 wrote:
"""So, decided to take another look at the carb and the main jet size as that's what controls the throttle range I'm having problems with right?""""???????????


Well there is the tapered needle position in the main jet, also the fuel level height a bit to high. Or maybe the exhaust system is partly blocked and limiting the out flow??????
I'm happy to give anything a try really, although carburettors scare me a bit Laughing

I've got a Haynes manual though so I'm guessing checking the jet needle height and float height shouldn't be too scary, especially as I have to take the float bowl back off to change the main jet anyway if I decide to try that.

With regard to the exhaust, how can I tell if it is blocked? Take it off and shake it? poke something down it? Any advice greatly appreciated.
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mpd72
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PostPosted: 09:45 - 15 May 2019    Post subject: Reply with quote

Yes, too rich on the main jet or a worn needle will cause it to bog at high throttle until you back off a bit.

I take it fuel flow is good and this isn't fuel starvation then?
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Steve_W
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PostPosted: 11:02 - 15 May 2019    Post subject: Reply with quote

mpd72 wrote:
Yes, too rich on the main jet or a worn needle will cause it to bog at high throttle until you back off a bit.

I take it fuel flow is good and this isn't fuel starvation then?
That's my exact symptoms there that you have described. Up to half throttle or thereabouts appears to be fine, but opening it right up from there to full throttle and it dies until you back off, then runs fine again.

As mentioned, last weekend after changing the coil and plug cap I ran it in my garage and kept throttle open until it died completely. I removed plug at that point and it was soaked in petrol.

I think fuel flow is good, it flows straight out of fuel tap no problems and there was plenty of fuel in the float chamber when I removed it. Is that enough of a "test"?
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Steve_W
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PostPosted: 21:22 - 15 May 2019    Post subject: Reply with quote

I haven't had chance to look at it again yet, but if I look at the Jet needle and it's set as it should be in the middle groove of the three (according to Haynes), is it worth moving the clip to the higher position to put the needle further into the Main jet to see if that makes a difference before I start changing the main jet to a smaller one?
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Easy-X
Formerly known as
HardlyDavidson



Joined: 08 Mar 2019
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PostPosted: 03:01 - 16 May 2019    Post subject: Reply with quote

The general principle of a carb is to have a pilot jet that's basically "on" all the time and supplying most of the fuel at ~20-30% of revs. As you twist the throttle you're pulling the needle out to allow fuel from the main jet to enter the system. At redline most of the fuel would be coming from the main jet (relative to how much is coming from the pilot.)

If you're opening it up and it bogs then the A/F ratio is off - it sounds like it's way too rich if the spark plug is saturated. You can try moving the clip and see what happens but also consider what Nobby said: if the A/F ratio is too rich you can either take away fuel or add air.
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mpd72
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PostPosted: 08:32 - 16 May 2019    Post subject: Reply with quote

Steve_W wrote:
I haven't had chance to look at it again yet, but if I look at the Jet needle and it's set as it should be in the middle groove of the three (according to Haynes), is it worth moving the clip to the higher position to put the needle further into the Main jet to see if that makes a difference before I start changing the main jet to a smaller one?


If the main jet is larger than standard, put the original size back in. It really does sound too rich to me. Lowering the needle may help, but I've seen a bike where the bloke spent months trying to get it right and it turned out to be worn needles letting too much fuel in.
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Steve_W
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PostPosted: 09:00 - 16 May 2019    Post subject: Reply with quote

Thanks all for your replies, it's much appreciated.

I'm quite good with my engine theory, I'm just not the most practical person in the world, hence why I decided to go into IT after failing my BTEC Engineering 30 years ago Laughing

I have always thought this was fuel related rather than electrical, it's just that the bike shop has sent me down the electrical route, so I've been replacing bits and cleaning earths etc but so far although it starts much better now, my problem is still there and the only thing left to change really is the stator plate - and because that's quite involved I thought I'd take a look inside the carb and see what's what.

That's when I discovered the 105 main jet, which according to Haynes IS the correct size for my model (GP100UD - finally worked out which model I have after much googling!).

The thing is, every other GP100 lists a 97.5 main jet size, in the same model carburettor, with the same needle and pilot jet.

It's all a bit confusing (but weirdly I am enjoying trying to fix it!).

I get that with 2 strokes it's always better to run a bit rich rather than too lean, so if I do change the main jet and it seems to fix it while running it on the stand, I would only take it for a very short run on the road before bringing it back and inspecting the spark plug, to avoid prolonged running if it's then too lean Thumbs Up

How can you tell if the needle is worn? Or is it just a case of "replace and see if it fixes it"?
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Easy-X
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HardlyDavidson



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PostPosted: 09:52 - 16 May 2019    Post subject: Reply with quote

ofc the other arrangement for having a rich A/F is the choke. Double check that's okay when you need a distraction Smile
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Steve_W
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PostPosted: 10:02 - 16 May 2019    Post subject: Reply with quote

I think the choke is OK.

What I mean by that is I only really use the choke for a few seconds when it starts from cold and it idles faster as you'd expect it to, then idle drops down when it comes off choke and I rev it gently a bit to warm it up rather than keeping it on choke.

I'll take a proper look at it when I have it all apart over the weekend though Smile
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Steve_W
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PostPosted: 14:04 - 16 May 2019    Post subject: Reply with quote

So I sent an email to a different bike shop in my home town because they run an online parts website (I didn't realise it was run by this specific company until I looked into it) to see if they could get hold of a new needle.....

I explained my symptoms and that I was looking for a Jet needle to fit my bike (using the part number on their website even though it was listed as "contact us" - a lot of other sites have already said it's unavailable but thought I'd try).

Their response has made me question whether they have a clue about what they're talking about to be honest (or whether they have bothered to read my email at all) -

one of their technicians has said it could be a blocked main jet and if I bring it in to them they will have a look through their box of jets to see if they have one that fits - no doubt at an extortionate amount of money!

Nah, think I'll just carry on myself and learn something in the process. The carb was cleaned by another (one man band "enthusiast") bike shop that I trust a WHOLE lot more than this particular dealer anyway. AND, if it was blocked surely that would give starvation symptoms, not flooding, right?? Rolling Eyes
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mpd72
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PostPosted: 15:54 - 16 May 2019    Post subject: Reply with quote

Steve_W wrote:
So I sent an email to a different bike shop in my home town because they run an online parts website (I didn't realise it was run by this specific company until I looked into it) to see if they could get hold of a new needle.....

I explained my symptoms and that I was looking for a Jet needle to fit my bike (using the part number on their website even though it was listed as "contact us" - a lot of other sites have already said it's unavailable but thought I'd try).

Their response has made me question whether they have a clue about what they're talking about to be honest (or whether they have bothered to read my email at all) -

one of their technicians has said it could be a blocked main jet and if I bring it in to them they will have a look through their box of jets to see if they have one that fits - no doubt at an extortionate amount of money!

Nah, think I'll just carry on myself and learn something in the process. The carb was cleaned by another (one man band "enthusiast") bike shop that I trust a WHOLE lot more than this particular dealer anyway. AND, if it was blocked surely that would give starvation symptoms, not flooding, right?? Rolling Eyes


I've bought 2T bikes which were running like crap and found carb parts missing, such as emulsion tubes.

If you want spares, what carb is fitted? Is it correct? Allens Performance will probably sell parts for it.

You could try another carb and cable for 40 and clean it yourself. It;s not hard once the float bowl screws are off.

https://www.ebay.co.uk/itm/113640737602?ul_noapp=true
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A100man
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PostPosted: 16:31 - 16 May 2019    Post subject: Reply with quote

If you need carb spares then go to a specialist like Allens performance or NRP. You won't get any joy (well unlikely) form a local generalist
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Easy-X
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PostPosted: 19:31 - 16 May 2019    Post subject: Reply with quote

Lol... one of my old neighbours used to drive a van with the following painted on the side:

"Car Mechanic: specialist in all makes of vehicle"
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Steve_W
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PostPosted: 10:25 - 17 May 2019    Post subject: Reply with quote

The Carb fitted is a Mikuni 39240, which is the correct one listed on CMSNL, Fowlers and all other places I've found that list them.

I can't see that model listed on Allens, but I have found a carburettor repair kit on wemoto for 25 incl delivery

https://www.wemoto.com/bikes/suzuki/gp_100_ud_ul_drum_brake_model/79-92/picture/carburettor_complete_repair_kit

That's about the only way I can see that I can get a new needle, although I guess I could call Allens, which I may do later if I get chance (I'm at work and supposed to be working now, but have spent a bit too much time over the last few days trawling the web searching for clues! Laughing )

I agree with you both about specialists and "specialists" Laughing

I didn't hold out much hope of the second bike shop being able to help me to be honest, they're main dealers for most makes, but I don't think they're in the same league as my first guy (the one man band guy that services, MOTs, restores and custom builds stuff) when it comes to older bikes like mine - but it was worth a try to see if they had anything in stock. I certainly wouldn't take my bike to the second one to be worked on anyway.

I'm going to work my way through a few more things at the weekend. It's a slow process with me as I never seem to get enough spare time, driving the kids about all over the place etc etc.


    Check needle and float height
    Check air/idle screw (can't remember if that's it's proper name) adjustment
    Change to a smaller main jet (I have 3 different sizes I've ordered) and see if it helps (if not revert back to original). Bearing in mind I will only run it on the stand to see if it runs better than before and if it does will take for very short run and check plug, as I don't want to run too far if too lean!
    Check exhaust to see if any blockages


After that if it doesn't fix it, I may have to buy the carb kit linked above and then it's back to changing the stator plate I guess, as suggested initially by first bike shop guy.

Incidentally, in my trawling that I mentioned earlier, I have found a lot of threads on dirt bike forums that show that a bad stator can cause similar symptons to the ones I am having.

I'll get there, eventually Smile
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mpd72
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PostPosted: 11:14 - 17 May 2019    Post subject: Reply with quote

That Wemoto kit has most of the important bits so could work wonders.

The only word of caution is that I used their kits once to replace the float valves on my TZR250 race bike and they wouldn't seal up and kept pissing fuel. On inspection, the float valves are visibly shorter so never sealed. In fairness I sent a photo and they refunded me immediately.

I ended up going to Allens Performance and bought 2 sets of float valves and cup kits. Expensive but did the job.

For 25 though, it has many of the parts needed to help the carb. Just make sure you clear out all holes and passageways too.

Also, don't forget a blocked or restrictive air filter will cause it to run rich too.
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