Engine sudden rev issue

Hey guys,

I was hoping you might be able to point me in the right direction with this problem.

I have a '84 vf1000 (60.000km), which I’ve purchased last year. The conditions isn’t great, but still a fun bike to ride. It ran great, but now I’m starting to have issues. When it’s cold, it runs fine but when it gets warm, it starts to have issues.

When standing still and running it at 3000rpm it suddenly jumps to 6000rpm without changing the throttle. While driving it gets even worse, jumping around in RPMs , so it’s like riding a mechanical bull!

I’ve replaced the ignition coils, because the old ones had cracked, replaced spark plugs (and leads) and removed the carburettors (but they seem clean). I’ve also checked the fuel pump, replaced the filter and checked the fuel pump relay.

It all didn’t help, so do you have any ideas?

I’m currently doing valve adjustments, but I can’t imagine this causing the issue. The rear exhaust cams look a bit worn (see picture)IMG_20210425_145501 IMG_20210425_144823

I’ve also noticed an unconnected tube between the top of the rear carbs, should this be connected?IMG_20210425_132530~2

So, do you guys have any idea what could cause this problem and point me to where I should look next?

Thanks in advance!

Check the high tension leads. On one of my r’s the lead from the front left cylinder had worn through the insulation against the frame. Did some very strange things to the idle.

Have you replaced the rubber carby boots as an air leak may cause that sort of issue also.

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Hey thnx113,

Thanks! I read that too, about the high tension leads, so I’ve replaced the high tension leads together with the ignition coils, spark plugs and spark plug caps, unfortunately that didn’t make a difference.

I haven’t checked for air leaks yet, so I will definitely do that. Wouldn’t a air leak also cause it to go back to idle very slowly after revving? (Mine returns to idle just normal)

Some of our American friends have had problems with intermittent problems with their Spark Units like dry solder joints. If you are confident you can actually pull them apart and inspect for dry or dirty contacts. A good clean and resolder all suspected joints (or all joints) quite often fixes those problems.
I only recommend this as you seem to have tried almost everything else. Classic Honda VF - V4 Sabre Magna Interceptor (honda-v4.com) Try this site and look for Ignition repair under Misc bottom right corner. Should tell you everything you want to know. There is other good info about all the Honda V fours as well


Thanks man! And thanks for the extra information. Will tear the spark units apart next weekend, to see if that helps!

So I’ve pullend the igniton boxes’ covers off and checked the solder. Looked fine, one or too very little crack as described, so resoldered them. Must see if it has any effect when I put it back togheter.

I’ve also measured the pick up coils, and noticed that one of them has infinite resistance (other one 480-ish). So I thought, that could be the problem. So, I’ve ordered new ones (second hand though), but they are exactly the same. One has good resistance, other infinite.

Could this be correct? Am I reading the instructions wrong? Anyone got a clue :slight_smile: ?

Thanks in advance!

Infinite resistance is bad, they should be the same resistance 480 ohms is about right, I’ve not got any to hand to measure but I have some in work so,will measure them on monday. The usual problem you get with failing pickups is stalling and only running on two cylinders not revving too high.


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I’ve recieved a second set of pickups from the seller. He measured them before selling and both were fine, but again I’ve recieved one broken :frowning: So, I’ve decided to combine the broken pairs to create one working one… Replaced it last week and it seems to work great again!

Thanks for the help :slight_smile:

It may have a low speed jet clogged. Then you’ll have to adjust idle screw in to get it to idle. Problem is that you now have gone into next stage on carb function causing a sudden increase of rpm. Carb with problem now running on a clean circuit.

Try not to lose focus of the stair steps of your carbs.

My vf also started to not rev over like 6.000rpm. Turns out the problem was the Carb Slide Diaphragms. New they look like some kind of fabric but when i opened mine they had become like chewed gum and liquid. replaced them and bike reved again high. It’s those 4Pcs Carb Slide Diaphragms Honda VF1000F Interceptor VF1000R 1984 1985 1986 | eBay