magna vf 750 wont rev.

morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

1996 honda magna vf 750 won't rev past 1200 rpm.
the bike started running rough last fall. I installed a set of plugs, and they helped a little. The bike sat up on and off for the last three months or so, but was cranked now and then and allowed to warm up. the running got worse so i poured a couple of oz. of sea foam gas treatment in it. but that didn't help. I found that the 2 rt. side cyl.'s werent hitting, so I thought it had a gas problem. Both the exaust on that side was cold. I checked the plugs and they were firing. I drained both the rt. side bouls and blew them out, along with all the gas lines. There was nothing in the bowls but clean gas. So I installed another new set of plugs. No help. After that I dumped the fuel and poured in fresh. still no help. It will rev up to about 4500 with the choke all the way out.

Any Ideas? Morris
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Odie · World Chat Champion Joined: 24 Jul 2004 Karma :

You might have perished carb diaphrams, check them for cracks / splits / pin holes.
Any of the above will cause it to run crap.
Odie
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Bru · Trackday Trickster Joined: 01 Feb 2007 Karma :

Have you checked that your plugs on the non-firing side are wet with fuel? That way, you will at least know that the carbs are spitting out some mixture without the choke.

Does your bike have two fuel hoses to the carbs? Possible blockage at the tap or hose?

If you have spark, then it's not necessarily coils, although you may still have spark breakdown under load..

hmmmnz · Super Spammer Joined: 22 Aug 2006 Karma :

i would pull the carbs give them a good going over, clean out all the jets, check the diaphragms, ect ect,
dump all the old fuel and put fresh stuff in,
and give it another go, make sure you are getting fuel to the carbs and make sure your vacume hose to the fuel tap is connected and not perished,

while the carbs are off give them a balance by eye,
eg the butterflys open and close at the same time,
i use a thin bit of wire or .5mm drill bit to set them .

eg place the wire in the mouth of the carb under the butterfly and close the butterfly until it lightly touches the drill bit, set all 4 the same,
then close them right up,

you should be able to start it on just the choke alone, well i can on my vf1000
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temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

Are the two dead cylinders fed from the same coil?

If its anything like the earlier VF750s, it has TWO ignition boxes. They arent of great quality and are known to die.

Check the coil (swapping them over) and then swap the ignitors. Check the assosciated wiring as well.
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morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

I thought the coils might be weak, but the two cylinders that are affected are run on different coils. I drained the fuel and run the bowls dry, and filled them with full strength B-12 and let it soak all weekend. I ran the B-12 out today and put frest gas in. It will rev now, but only the two lft clyinders ar hitting , so I've got it back to where I started. I begining to think that maby it ha back fired a some point in the past and blew the 2 rt. side main jet needle diaphram's. thus not letting any fuel in on those 2 carburators.I don't know of any way to check them without pulling them , and it looks like it will be a nightmare. the carb. bowls have been full every time I drained them , and the plugs aren't wet. The exaust is still cold on that side. anyone ever had these things apart?
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morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

Here I am again!!! I checked all the vacuum lines again this evening, this time with the carb. stack plate off, but didn't find any problems. I was able to get the # 1 & # 3 main jet diaphrams out , but they were perfect. I sprayed B-12 through the vacuum port, and the main jet, both seemed to be clear. Tomorrow I will drain the bowls again and see if anything might have washed out.

This thing is about to get to me.
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Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

If you are getting both left cylinders running it kinda rules-out an ignition fault, since as you say they are run from separate ignitors and coils. Points to a fuelling problem, but being a VF I'd have the cam covers off and make sure the cams still have lobes on them. Is it unusually noisy at the top-end?
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morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

There is no noise from the top end at all. The bike only has 32K on it. I didn't get a chance to drain the bowls on the 2 cylinders today, but I'll try to get in a little earlier tomorrow.
The manual mentions that the EGR system could cause some problems , But I havent figured out how it works yet.
To my knowledge, all I need is fire, gas,& vacuume to pull it pull it into the cylinder . With only three variables, A fellow should be able to figure out which one is missing.
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morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

I finally got the carbs. back together, and it is now hitting on all cyl.'s. BUT, it still wont rev. , wont run past an idle unless I pull the choke. all the fuel system is clear and clean. Any ideas?

Morris
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temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

If its survived this long the top end has most lkely has the oil feed sorted.

Are the plugs sooty? or white?

Id check fuel flow from the tank, in case its starvation. Also change any fuel filters if fitted. I'd also carefully inspect the fuel pump. I replaced the one on my VF to a Facet pump as the original died.
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morrisliv · L Plate Warrior Joined: 18 Mar 2010 Karma :

OK guys, I finally got time to work on the bike again, So I bit the bullet and pulled the carbs. they were like new inside. no dirt sludge or anything. The floats and needles worked perfect. I pulled the jets , and they were clean also. I blew out the fuel lines, and they were all clear. So I put it back together and am now waiting on a new air filter to arrive.
I ohm'ed out the coils and pulse generator and found that the coils were rite on , but the pulse gen. was at 300 ohm's. It should be from 400 -- 450. Does anyone have experiance with these parts?

Thanks Morris
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