*POP* and Hesitation/Flat Spots

[Ingah]

Well that went well...

Having spent the last few days fighting with my carburettors, engine manifolds and airbox (along with masses of jubilee clips), i finally lost my patience with the bike and sent it to the garage for them to finish off the job (i.e. take the carbs off yet again) after i realised i'd trapped the choke cable in the left switchgear and the accelerator/deccelerator nipples were connected the wrong way round (so that my throttle was operating in reverse) - i'd done everything else by this stage and was/am sick of the whole shebang.

Note: My restrictor kit consisted of restricted engine manifolds and a jet change (#122 to #125 for restriction - that's bigger jets for restriction as #122s are standard, but i didn't question Honda on that point).

I picked the bike up earlier, and unfortunately found that it no longer accelerates properly! The bike is fine provided i only open the throttle part-way, and don't go above 5/6K RPM (10.5K redline) - i.e. ride it like my CG125. But with a large throttle opening and/or higher revs, it makes a noise that reminds me of the inrush of air, and accelerates quite slowly, almost hesitantly, like a big flat spot. It also makes the odd popping sound, indicating it's running lean.

I took it back immediately, and the mechanic manage to adapt a jubilee clip to fix the one that was missing between the carbs and the engine block (my fault, i'd broken it during my exertions), as he surmised this could be the problem. This didn't solve it though

I'm thinking it's an air leak somehow (air leaking in), but i did partially take the carbs apart myself whilst changing the jets, so it's very possible there's been a mistake there on my part. I'm bitterly disappointed it's not working as i tried very very hard to do it myself initially and have the skinned knuckles to prove it!

The mechanic is next available on tuesday, and i'm hoping i don't have to take it back as it could well take a while for him to troubleshoot, which will hurt my wallet even more. That and it's my main transport! It's now a little too late to have never started the job in the first place
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-- Ingah

[stinkwheel]

To check for an air leak. Start the engine, leave it idling and spray some EZ start over the inlet rubbers. If the revs increase when you do so, you have an air leak.

You can also use WD40 but it doesn't give such a noticeable rise in the revs.

Did you have the tops off the carbs? Could you have torn one of the diaphragms?

Kats CB500 is running on washers and 125 jets and it revs all the way to the redline in the first three gears. Starts running out of puff at higher revs in taller gears, which is pretty much what I was expecting.

You do know the airbox pivots back once you've taken the battery out yeah? I had no issues with space to work in. One of the easiest carb removals I've ever done on an inline engine. I didn't even take the thorttle cables off, just pivoted the airbox back, popped the carbs off the inlet manifold and rested them on the front of the frame while I tinkered with the rubbers.

Hardest bit was remembering where all the rubber drain/overflow hoses went, but that's why we have digital cameras.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[nightshaddow]

you could try raising or dropping the needles, i had a similar problem with one i did recently i raised the needle to the top and it run better (had to put bigger jets in to get it right but honda provided yours so "should" be right)
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"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[Ingah]

[Ingah]

[temeluchus]

[Ingah]

On another forum it has been suggested that the carbs could now be out of sync (this is assuming that derestricting can mess it up - i did get them synced about 6 months ago, whilst still restricted of course).

Is this a wise thing to check second (after eliminating air leaks from the equation)? - It'd be a pain to check you see, as i don't own the gauges, am getting short of cash, and don't want to buy some cheap naff ones (i believe strongly in "buy right, buy once" as far as tools are concerned).
____________________
-- Ingah

[stinkwheel]

I doubt out of synch carbs would cause what you describe. More an uneven idle, slightly rough running and clattering noises from the primary drive. On wide open throttle, the butterfly valves are fully open and the main jet is fully uncovered. Balance makes little to no difference at that point.

Definately check that diaphragm (inspect it really closely, even a pinhole will totally screw it up).

Don't worry about unfiltered air. I know plenty of people who run bikes with bell mouths on the carbs and some of those have been running since the 1950's. It wont carburate properly without the airbox and filter but a lack of filtration, especially for a short time, is not going to be a major issue.

Might also want to check both throttle needles are attached to the throttle bodies. There isn't much holding them in. If you had the top off one carb and didn't get the needle square back down the jet hole, it could easily have been pushed up out of place and just be rattling about on top of the jet. Would make that side run lean as hell, I've done that myself on an old CB125. Took me ages to find.

I should clarify. There were a couple of bolts to remove on the airbox before it pivots back. I used a bungee to hold it back out of the way.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Ingah]

Hold up just a second - when i had the carb off the bike i gave the whole unit a good solid spray with carb cleaner. Does this affect things? (if so, it's a "doh!" moment).

Also for more clarity, when i open the throttle over about half-way, it misbehaves (fine before then), and so i can't get more than ~60mph out of the bike otherwise it's clearly struggling (jerks abit, just like my girlfriend's car did when her lamda sensor on her fuel injected fiesta had broken). I believe this half-throttle is the point the main jet comes into play?

I've decided to test for air leaks myself as it only takes a minute (have reserved some "Easy Start" from Halfords, to spray over the outside of the engine manifolds), and if this doesn't make it clear (i.e. that the new jubilee clips will solve it), then the garage can have it again (later.... today, god is that the time already? ), with instructions to strip and check the carburettors (especially the needle and diaphragm). I urgently need to study so i can't afford to faff with the bike any more than i have done.
____________________
-- Ingah

[Ingah]

Well, there wasn't an air leak.

However, i'd made 2 mistakes with the carburettor.
1) Somehow not secured the main jet on one side correctly so it had fallen out and was loose in there! (Explaining lack of top-end)
2) During my attempts to disassemble the carb, i had unscrewed a screw and found it led to nothing, so screwed it back in, tightly. This was the air/fuel mixture screw on one side of the carburettor, and explained the lean running while idling etc.

So red face for me - lighter of pocket too (as repairing my bodges at the garage cost me more than if i'd just given it them in the 1st place), but lessons have been learned
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-- Ingah