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84' Vf700F, Engine Cuts Out Under Acceleration


ScottJ

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Hello all,

I’ve been seeing a few posts lately about bikes stalling out under different riding conditions. I hope it’s OK if I submit another similar post but with different conditions. The bike starts, idles and runs great; it’s only when it reaches a certain engine temperature that the problem occurs.

I have a 1984 VF700f and the bike runs fine until the temperature gets just past the operational mid way point on the temp gauge, but certainly nowhere near overheating. The problem is that the engine wants to die away when I start to apply throttle from a standstill (the engine is still idling normally). I have to rev the engine to get the RPM’s up close to redline and slip the clutch to get the bike in motion. At that point the engine will sputter and miss but will then proceed on down the road. Hopefully I’ll be able to ride it a ways to get the temp to come back down but, it’s usually in stop and go situations that I encounter this problem.

My first thought was that it might be a carburetion issue and it was loading up the plugs. I’ve since had the carburetors rebuilt so I’m thinking that might not be the problem. I’m now leaning towards the ignition and all the components that make up that system.

I have to admit some ignorance here, I’m not completely sure what the igniter boxes do as far as the ignition process but, I think that if one or both of them were bad that I wouldn’t get any spark at all. As it is, it seems like I’m still getting spark but that it is very weak (but, this is just an assumption on my part).

As of today, here’s what I’ve done to try to correct the problem

Removed carburetors and had them cleaned and rebuilt. Billy Carr did the work and I was very pleased with the results. He said the settings have been returned to factory spec and since reinstalling I’ve had them synched but have done nothing with the air/fuel mixture adjustment. Fuel pump delivered the required 21 oz.+ per minute. Replaced the 4 carb boots as they were pretty petrified. The bike, being about 30 years old, all of the above was probably overdue.

Spark plugs are only about a year old and don’t have more than about 1K on them. NGK plugs in the recommended range.

New aftermarket rectifier/regulator installed about 100 miles ago.

The motorcycle has very few modifications, only a K&N air filter and a Hole Shot top end oiler.

Also, over the last couple of years I’ve adjust the valves about three times. The first two times I used the procedure as described in the Clymer manual and adjusted at the tappets. This last time I measured at the cam lobe and rocker pad (Dave Dodge method?) and that seemed to work better. I’m satisfied with the results (but not extremely satisfied). Still some noise but I was told that if you can’t hear a little noise from the top end, you’ve got the valves adjusted too tight.

That would leave the coils but I hate throwing parts at the bike hoping something might work. And are there still good, functional coils out there to be had? If the problem was more consistent it would be easier to isolate. But, the bike runs so good up until it reaches that mid operational temp and then the bottom just drops out.

Hoping someone might have some ideas or suggestions they can pass along. As above, I can turn a wrench on the bike but when it comes to the electrical, I’m a Neanderthal…

Thanks to all for reading this, I know I’ve gone on probably longer then I’ve needed. Any and all suggestions/ideas will be appreciated.

Thanks again

Scott

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A possibility is thermal breakdown of the pulse generator(s). Usually a bad pg will allow good spark when cold, but fails completely when it gets hot. I haven't seen a pg failure cause intermittent spark, but I suppose it could happen. Food for thought.

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I had pretty well the same problem on my 84 vf750f and it turned out to be the

pulse generators. As it got to halfway or higher temp the same problems occurred.

The FSM has the instructions to check and I don't think it was expensive for parts.

Garry

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Here's how to check the pulse generators. The pg plug is located on the right side of the engine between the rear cylinder bank and the frame. Uncouple it and probe the wires: probing the white/yellow and yellow wires (cylinders 1&3) and then the white/blue &blue wires (cylinders 2&4) should give a resistance reading of 480 ohms or thereabouts. You will probably have to warm the engine up until the problem shows itself to get an accurate result.

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Creaky and Lobotomy (I hope I can call you Lobotomy), Thank You for your input. As both of you indicated that the problem is likely to be the pulse generators, I'm going to give them a serious look. I've got an ohm meter around somewhere, I hope it still works. But I did take a look at the wiring harness/plug this evening. The five wires coming out of it are blue, yellow, black (with two small plastic rings of green and red... I'm assuming this is to the oil pressure warning switch) and two white. I've examined these two white wires and can find no trace of blue or yellow on them. I'm hoping that they grouped them logically on the plug, which would be next to their counterpart and not diagonally across. But, probably not a big deal and I'll cross that bridge when I come to it. I checked my usual parts source and found that the pulse generator is now a discontinued part. A brief internet search revealed the same. I'm assuming that I'm going to have to have some luck and order them off of eBay and hope I get a working set? Would anyone be able to tell me if I have a range of years that would work or am I limited to '84? (also, am I limited to the 700 or will a set from a 750 work as well?)

Again, Thank You... this issue has dogged me for the last couple of years. Would be great if I could get it sorted out.

Scott

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I was able to borrow a meter from our building engineer here at work. Bringing the bike up to the temp where it starts to cut out, I was able to take a reading at the connector plug which showed 662 ohms. This is not what I was expecting as I thought it would show something under the listed 480 ohms. Does this still indicate that the pulse generator is indeed bad?

Thanks again

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  • 2 weeks later...
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I'm goin to post this question on the V4hondabbs.com forum. Due to a high volume of spamming the administrator has made it difficult to register an account there....please stand by.


The feed back I'm getting is most likely the pulse generator but it also could be a coil failing when getting hot.

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  • 2 weeks later...
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JeffInFranklin, Thanks much for submitting my question...I appreciate you taking the time to do this. Seems the pulse generator is getting the most votes as the likely culprite.

I found one on ebay and have ordered it. I'll test it at work and the seller said he would refund my payment if it doesn't check out to spec.

Again, Thanks to all who have speculated and made suggestions, this has helped me immensely to narrow down the problem.

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