Hard starts, high temps and weird idle (Bonus question included!!!)

I have the following (possibly related) issues with my 1984 VF750F:

Hard Starts - I recently had to replace the carbs due to a cleaning going TERRIBLY wrong. Upon cleaning the replacement carbs, it appears that the main jets may have been replaced with an upgrade??? The bottom of the jet was a separate piece screwed into the rest. Sorry for my poor description. Can't find the right words here. ANyway, the cleaning of these went pretty well and every port and tube was cleaned and verified clear before reassembling. I used spring wire to verify that each passage was clear and also blew carb/choke cleaner through each one to double verify.

After reassembling the carbs and re-installing on the bike, I find that the bike is very difficult to start, but only when it sits overnight. Definitely needs to be choked. It takes several attempts to get her going. First time nothing. Second time a few rotations before stalling. Third, very poor idle then stalls after a few seconds. Fourth, poor idle but keeps running and eventually messing with throttle and choke evens out and runs normally.

High Temps - The temperature on the bike seems to be kind of erratic. Sometimes it sits around the first third of the gauge. Sometimes it will sit in the top quarter. Always lower when moving though. I've seen allot of complaints about the thermo-switch for this bike and that most install a manual switch for the fans. Is this my only hope to remedy this?

Weird Idle - After the bike reaches higher temps (anything above the halfway mark on the gauge), I notice that the idle sits at around 3k instead of 1.5k. Since it follows the temp, I am assuming this is related, yes? Any ideas on how to correct?

BONUS QUESTION!!! - After riding the bike for an hour or so and shutting it off, it will not start. The engine cranks and I hear it randomly try to catch, but no go. If I let it sit for 15 minutes or so, it will start fine. Could this be related to the above issues?

Thanks in advance for any input, help or other outbursts that you may have to offer!!!

 

Does it perform normally under load once it gets running? No stumbling upon acceleration, no loss of power in any specific range, etc.?

 

Sounds like fuel delivery problem to me? Fuel pump, filter, float level or petcock? Good starting point anyways.

 

Runs good after its running. Actually, the idle creeps up to about 2.5k when the bike gets hotter.

I know there is some issue with the petcock as the bike will run with it in the off position.

Filter could also be a culprit. Fuel pump seems strong.

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Never forget the common blasted treachery of the fuel pump RELAY ! They fail more often than pumps. :distrust: Did you clean out your fuel tank and check for signs of rust ?

One explanation for hard starting could be low fuel level in carbs, lost after sitting and evaporation, especially in hot weather. Same thing with any Honda after weeks or months of not running. So you need--because of a flakey Honda design--to crank and crank on the starter, drawing major power, before the carbs are topped up by pump. Then battery reserve may be insufficient, and again, no start. One solution is to wire in a bypass connection on the relay plug so the pump has 12+ power whenever the key is on so carbs are ready to go first push.*** When restarting and the engine is hot, it often helps to hold the throttle open just a bit when cranking.


Next time it won't start, before waiting a few minutes, start pulling plug wires and check for healthy, snappy, bluish, big sparks. Don't bother removing any plugs, uze the spare you carry on the bike. One component subject to failure when things heat up is a pulse generator, and we have 2 or 3 --if they're beginning to go bad. Simple resistance tests are described in the FSM.

Fan and thermoswitch are both OK if fan kicks in at just about 2/3 of the temp gauge scale.


** Yes, frequenters of this site know this suggestion as my FAVORITE mod to almost ANY Honda bike.

 

Would the bike run at all if the pump relay was bad?

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Tell us more about this? Do you just bypass the relay all together and leave it like that? Will this cause the pump to pump itself to death? Why is the relay there to begin with?

Not questioning you, just would like a better understanding of how this works so I can look into doing it myself. Something is up with the pump or filter on the bike I just got...wont run with the pump/filter, but will run when bypassed and just gravity fed. This was with a full tank.

 

Getting too controversial for me now, hahaha. Relay may be in place to satisfy USDOT safety requirements or cuz Honda lawyers insisted. It's useless but of course could (in one accident out of 10,000) save you from a firey end if you crash and forget to switch the key off. "What?? Motorcycles dangerous ??" It works great but may not be the best mod for those obsessively risk-averse, fathers with 12 kids or riders with pink helmets. Pump shuts off normally as soon as it senses carbs are full, relay or no.

A hot engine always idles at higher speed than a cool one (with carbs), and the idle speed adjustment screw is there to compensate. Use it.

 

Not trying to hijack your thread Eric, figure this info is interesting/useful to you too.

The way I understand it the pump only pumps once per engine revolution upon signal from black box. Is it the relay that takes the signal from the black box and then completes connection to battery so pump pumps, but then right back off and so on?

Take the relay out and just go straight to the battery and boom....pump pumps when key is on. I didn't realize the pump had a pressure switch, I thought that's why it only pumped once per revolution. But if it has a pressure switch then it sounds like an easy choice.

Where exactly does the bypass connection go? Sorry I sound like a total noob here but I am with this bike. Right now I have the entire fuel pump/rectifier panel pulled out so this should be easy to do and test on the bench with an extra 12v bike battery I have.

 

http://vfrworld.com/forums/showthread.php/45608-85-VF700-Fuel-Pump-Relay?highlight=relay+bypass

Found a thread that answered my questions....unplug the relay connector with three wires, jump the black and white wires and that's it. Leave the connector unplugged and run it, probably a good idea to tape it up. Now the relay just sits there all dumb like never knowing what happened.

I just ran my fuel pump out of the bike and on the bench, wired direct to a battery....it pumped its little heart out....went through a gallon quick fast and in a hurry.....through the stock filter too.


To get back to the OP question...the main jet is supposed to be a separate piece from the tube with the little holes in it. The main jet is just a tiny little thing that screws into the tube.

I vote you jump the relay to be sure you are getting fuel to the carbs, your symptoms sound like they could be fuel starvation to me. Even the high idle as the mix leans way out due to not enough fuel in the bowls.

Or it could be the other way around. An overheating engine will idle high, ride an air cooled bike in a traffic jam with no movement and you figure that out pretty quick!

But its will be easy to eliminate fuel starvation by just jumping the relay, maybe even put a piece of tubing on the bowl drains and make sure they are getting fuel. 3/16" I.D. fits mine perfect. Would probably even be a good idea to measure how much each bowl is holding and make sure they are the same. If I knew the designed fuel level I would say to use the clear tube method, but I have no idea a reference point on the carb to measure the level against.

 

Looks like I have allot to check out this weekend. Thanks for all the input. I'll post my findings as they arise.

Thanks again!

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Reference point is different if carbs are on bike or not due to difference in angle of fronts vs. rears. If off the bike and airbox top is leveled out, proper fuel level is exactly where lower float bowl meets top section of carb + or - 1mm on all carbs measured halfway between front and back. If mounted and the bike is absolutely level side-to-side, use that reference point for front carbs only as rears will be about 5-6mm different. I don't believe that measuring quantity of fuel in carbs provides much useful info since you're also measuring whatever residual fuel is left in supply lines too, and the amount held there may be different for different carbs.

 

So before tearing the carbs back apart, I wanted to see if the bike would pass inspection as it sat. It passed!

I asked the mechanic about the hard starting of the bike. His suggestion was to try a tank of gas treated with this stuff:

The result after just one tank was totally amazing!

The bike starts on the first try now and the idle has smoothed out considerably.

Next, I will probably flush the current coolant out and replace it with 'Engine Ice'. The radiator cap needs replaced also.



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The answer to all, including the Bonus, is in the carbs. You don't adjust the idle using the thumb screw--idle is adjusted using the mixture screws (did you pull those and clean?) The idle speed adjuster thumb screw is only used to slightly hold the butterfly valve open if necessary to keep the motor running while you perform the mixture screw adjustment. Someone has turned your screw too far to open the throttle and compensate for incomplete cleaning of the carbs. The butterfly valves should be shut at idle--all air and fuel bypass the throttle plate thru either the idle or 'choke' circuits. If the throttle is open at idle it will pull fuel thru the transition ports or the main jet instead of the mixture port and result in a 3-4k rpm idle speed.

Did you pull the emulsion tube for the main jet and clean all the cross-drilled holes?

http://vfrworld.com/forums/showthre...-Fuel-and-Air-Circuit-Identification-and-Porn

http://vfrworld.com/forums/showthread.php/22449-Cleaning-carbs