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hesitation when accelerating

stevew51

Forum Website User
Non-VOC Member
My 51 Rapide is experiencing a hesitation and power loss when accelerating in 3 or 4th gear. I've checked the magneto timing, valve adjustments, carb. jets, slides and tuning. Fuel flow is adequete to the carbs and all the fuel screens are clean. The ATD seems fine and the points are good. The bike runs fine up to about 40 mph then hesitates or bucks slightly above that. I removed the air filters and it got worse so I suspect fuel. I also reduced the front cyl.'s spark plug gap to .015" to see if that improved anything, it did'nt. It does not labor or conk out and it gets no worse or better when warmed up. Please advise.
 

stevew51

Forum Website User
Non-VOC Member
hesitation reply

It came on suddenly, while I was going down the highway. It had been running all that day, just fine. The bike has not been used except for short runs, since December. I always drain the carbs and remove the gas (petrol) from the tank when I am laying it up for any lenght of time.
Thanks for your reponse, Steve
 

Bracker1

Well Known and Active Forum Website User
VOC Member
Greetings, Check your valve lift mechanism. There might be slight tension on the exhaust lifter that could affect compression. Low speeds not so noticiably. We have junk fuel in the US, ethanol mix. I think my Ford engineer friends recommend BP fuels. Good Luck, Dan
 

Tnecniv Edipar

Well Known and Active Forum Website User
VOC Member
Ok , presuming all the stuff you checked is ok , try closing the choke slightly when the problem occurs , if it's better then carburation is the likely culprit. Another issue could be weak spark due to a mag problem , condensor , points gap closing up , dirty points etc. A weak spark will struggle to fire as the throttle is opened. I have no experience of US fuel so that indeed could be a prob , bad tank of gas.
 

stevew51

Forum Website User
Non-VOC Member
hesitation problem

I tried closing in on the choke, while it was hesitating and it made no difference on either cylinder. Although, removing the air filters did make a big difference (worse). I am going to try emptying the tank and go to a different station for gas, also I have not checked my gas cap vent yet.
The mag was just rebuilt last fall and seems to be fine. It starts easily on a kick or two after priming the carbs. The mags I've seen usually fail when hot and at low speed.
Thanks for your response, Steve
 

stevew51

Forum Website User
Non-VOC Member
Hesitation problem

I just saw your response, I did check the valves and they were fine. There was no slack, I just able to turn them with a needle nose pliers when the cylinder was at TDC, compresion stroke. The problem does not seem to get any worse as the engine heats up.
Thanks, Steve
 

Tnecniv Edipar

Well Known and Active Forum Website User
VOC Member
Removing the air filters shouldn't make a huge difference , but as they did it would suggest a weak mixture. Are both cylinders affected though ? Can't imagine the same carburation fault would affect both carbs simultaneously , unless it was common to both , so we come back to fuel again.
 

Comet Rider

Well Known and Active Forum Website User
VOC Member
Bio-Ethanol problems

Hi Steve,

If you go back through some of the other posts, you will see that many of us have had problems with modern bio-ethanol fuels.:mad:

It seems that the cheaper the gas (usually supermarkets or cheap mall outlets) the worse the problem. Please do try draining the system and re-filling with a quality brand for a direct back to back comparison.

Here in the UK the rubbish stuff comes from our local supermarket, which is now avoided like the plague. Even a local car dealer has now told any customer that their warranty is void if they use supermarket fuel:(

Best of luck
Neil
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
Hi Steve,

Have you tried changing the spark plugs. I had a similar problem last year which was cured by changing the plugs. What initially caused me to look else where was the plugs were only 500 miles old. It was only one plug that was faulty. They were NGK which I have used for years, but it has been reported on the internet there may be a problem with NGK quality or compatibility between NGK and the modern fuel in Europe. Not sure about the USA though. Changed to Champion plugs and no problem over the last 3000 miles.

Simon.
 
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