-83 VF750F front cylinders not running

Bike has been unused for last 4 - 5 years. Previous owner says valve clearances should be ok, they have been checked just before he left the bike in his garage. Only thing we have done is cleaned the carbs and changed the leaking fuel tubes. Now the bike is running quite ok with the rear cylinders but not with the front ones. We swapped the ignition coils but it didn't make any difference. Spark on the right front is a bit brighter than on the left. Idle mixture screws are in factory values now, 2,5 turns out.

I could get the front ones to work if I put some cloth in the front intake. By restricting the air, front cylinders fire up almost immediately. I tried to adjust the idle mixture screws but I couldn't get it to work without the cloth. Any ideas whats wrong?

Compression tester says front right has the weakest comp. Warm engine 140 psi, poured some oil in and it went up to 160 psi. Could this be a problem?

ps. I'm talking about we because I bought this bike for my girlfriend as a birthday present. She wants to do all the work, I'm only working as a advisor. Only plan is to get it running so she can drive her license with it. After that we have to decide between keeping,restoring,modifying,selling,parting.

 

Sounds like either the 2/4 ignition box or the pulse generator for the 2/4. Though you are getting spark which doesn't make sense cause you swapped coils and a bad coil should have showed up. Make sure the contacts of the 2/4 ignition box are clean. The 2/4 box is in the tail on the left with the red connector.

There is one other thing. The spark plug wires have a resistor in them inside the cap. They unscrew from the plug end. If you look inside you will see a slot for a small screw driver. Sometimes they get corroded up and need cleaning. If I remember correctly the plug wires should ohm out about 400 ohms with the resistor in them.

 

Grey this wouldn't account for the fact it fires up ok with a cloth over the front carbs. It is obviously a carby problem

 

Check for air leaks around the rubber carb mounts

 

Ok I missed that.about the rag thing Then those carbs are not right. Idle jet and or circuit is still plugged. Idle jet is a .012 wire to clean out. Don't use any thing bigger. Make sure the curcuit is clean from front of carb to behind throttle plate. There's several holes. Make sure to use a pressurized fluid to shoot though the circuit.

 

Thanks for the hints! We will try to clean the carbs again and check the rubber mounts.

 

We got it running. After ultrasonic washing the carbs, replacing two clogged slow jets and making new spark plug wires it is now running with all 4 cylinders. Carb sync also done. I tried to adjust the idle mixture like the service manual says but got no better result. It seems to run ok with factory setting 2,5 turns out.

 

This about the 289th time on VFRW where some bright lad :tickled_pink: claims to have cleaned his carbs but is unaware he didn't get it right. Then he looks for something electrical to blame. Time after time. The methods exist for testing carbs and parts b4 they're installed. :stung: Particulaly, idle jets can be flow-tested with a hose attached. Fill hose with some soapy water 4 or 5" high and time how long it takes to drain thru jet. Compare all 4. If the jet is clean you can force a thin, steady stream of water through in a pin-point concentrated flow without dribbling or drabbling, using lung power.

 

Not his fault...that's Honda's problem. Who would even put something like that on a bike? Flat out stupidest carb design I've ever seen, and I've worked on vintage BMWs for years. Ever hear VMax guys complaining about their carbs?

 

NO, no, DD; it's all instructive.

Just like the recent carb-cleaning work i got from a member here from Florida. I asked that he include the mounting boots.......and 4 out of 4 have serious seal damage from a partly-botched installation (easy to do). They show cracks and voids along a critical sealing surface and could cause air leaks.

You may find that you need to live with lowish compression cuz parts to fix it may not be available.

 

You are absolutely correct. During the first clean up I even tried to see through one of the idle jet but could not. I took another jet and could not see through that either. So I thought(always a mistake) that they probably should be that way =)

We actually had a genuine "electrical" problem also. One of the spark plug caps was so corroded that it broke in half when it was removed.

 

You have to love this site, don't you? Solutions available to all your Interceptor troubles...(almost) 24/7!

Glad to hear it's sorted out. Good to hear that you avoided the health hazards of carb cleaners too. Avoiding them with ultrasonics or other mechanical means is a great idea whenever possible.

 

New difficulties have arisen. Now when we got it to actually move it's working perfectly under 4000 rpm or low throttle. With high throttle over 4000 rpm it's not working well. If you keep throttle open it feels like there are 1 s pulses when it tries to accelerate. Top speed around 75 mph. Without load (neutral or clutch in) it revs nicely to 10 000 rpm. No misfire can be heard. Without the air filter I can see that the slides are opening when I blip the throttle. I have tried to move the fuel filter from before the pump to after. Bypassing the pump and gravity feeding fuel didn't help. Float height has been adjusted to 7 mm. Help?

 

Once again my own mistake. I re adjusted the float height and now it goes like it should.