1985 vf500f wont rev past 5k under load

Hey! Been lurking here awhile now, and figured it's time to cut the ego act and ask for some help.

I've got a 1985 vf500f that's been sitting a LONG time and I've been successful in getting it running and driving again. WOOO HOOO

What's been done to it:
Complete carb rebuild and cleaning.
Valve lash adjustment.
New plugs.
Oil change.
Coolant flush.
Compression test cyl 1=110 psi
cyl 2=105 psi
cyl 3=115 psi
cyl 4"105 psi
Fuel pump delete.
Petcock is now manual not vacuum operated, deleted vacuum lines for petcock.



What the problem is:
Bike starts super easy, and idles extremely well, if a little uneven (pretty sure my initial balance is a little of) throttle response in the low end is good.
BUUUUT, when the engine reaches about 5k (no tach sadly) it coughs and sputters and will not rev higher (it sounds like it's backfiring out the exhaust, no flames) This is under load and while driving. If I slowly roll onto the throttle it will rev higher but only till about 9k maybe 10k, and once the throttle is open past 50% it immediately coughs and drops to around 5k again.

I've messed with the pilot screws a bunch, but to no avail. The spark plugs all read a rich running condition.

Any help is extremely appreciated, thank you for reading!

 

Could it be the igniter boxes ???
My Vf1000f had the same issue and it was one of those boxes


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That's what I've been reading about, I'm looking into buy some used ones. Did you have any luck finding new units?

 

I bought some aftermarket o e off eBay from vfour spark


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Thank you! I'll look them up

 

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Let me try that again


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Wow! Now that's what I need

 

you know the pilot screws have NO effect at or around 5k rpm and mostly effect idle up to about twenty-five hundred rpm at one-quarter throttle, they need reajustment for sure

be sure all four vacuum slides move up when twisting the throttle. might need to check with the airbox top off while running

 

I have, thankfully all 4 open completely when I rev the motor. And I can see fuel flowing freely up from the main jet too, befor the valve lash adjustment the engine seemed to blow back out the carbs, but it stopped doing that after the adjustment. I've got the pilot needles set to the factory 2 turns out.

 

No worries Will .. with the Squirrel and a few other guys here you will have that bad boy on the road soon


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only a starting point, improvements can be made with some tuning

 

Good to know! It does stumble a little from idle, but over has good response and returns to idle well.

I'm going to work on the bike again tomorrow and go through the diag for the coils and cdi boxes. If anyone knows a good way to test cdi boxes, that'd be awesome! Thank you all for the help so far!

 

@squirrelman
Would you know if I can use the ignition coils from a honda sabre 1100? They don't have the built in ignitors, and look almost identical to the vf500 coils, but the plug wires come out the front not the side. I'm going to test the coils that are in the bike in the morning (night shifts are GREAT) and if they test bad I want to buy new units and not gamble on used ones.

 

So, just did the testing on the coils, the connectors going to the ignitor boxes were 2.4 and 2.6 ohms, a little low but close enough to the 2.8 spec.

The resistance across the spark plug leads however, are very low. 14.5k ohms when the spec is 21-28k ohms. I'm thinking this means that there is a short somewhere along the internal windings. This bike did sit out in the rain and snow for years so I would not be suprised if theyve rusted internally. The plug wires have good resistance readings, about 5k ohms each. And doing a spark test with the spark plug grounded on the frame, the spark looks small and weak, I don't think the coils are keeping up with the high rpm and can't produce a strong enough spark.

 

Well alright!!! Got very lucky in my search for ignition coils already!

Found some coils that are identical to the vf500f coils in every way I can tell.
I'm getting them tomorrow morning, so after my night shift I'll install them and update on what happens.

 

Cool...keep us posted

 

Noice diggin! I used to hate when my CDI would give me grief ( 2 x CBR600s). Used to think it was fuel related, but was just the box of electronic gremlins messin round.

I know...cool story bro.

 

The leads and caps (which is the culprit 99.9% of the time for weak spark) are removable on your coils. I have only had to replace a coil one time in the 35 years I've been working on Interceptors. And that was only because it had a huge crack in the body, it still worked.

I replace the leads and caps with high quality NGK items. The chances of both Honda coils going bad at the same time is zero really. And one failing coil would still likely allow the engine to rev past 5K.

I hope it's your problem and I'm wrong, but from what you describe, it is unlikely "bad" coils.