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hesitation when accelerating

stevew51

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

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 Website User
Non-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 Website User
Non-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

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

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 Website User
Non-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 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
 

vin998

Well Known and Active Website User
VOC Member
VOC Forum 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.
 

stevew51

Website User
Non-VOC Member
Results from tests

Thanks for the advice, I tried new plugs and new gas from a major brand gas station and I still have the problem. The bike is running better though.
It is acting like a car with a dirty fuel filter but it really should be getting plenty of gas accourding to everything I've checked.
 

Prosper Keating

Well Known and Active Website User
Non-VOC Member
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

It is probably a tankful of duff fuel or even some moisture in the float chambers. However, if you need a needle-nosed pliers to turn your pushrods, that's a smidgen tighter than I like mine to be. You should be able to get both forefingers in there and turn them quite easily but with zero up-and-down clearance. That said, some riders slightly increase valve clearances in relation to period factory specifications since the advent of unleaded fuel. I've tried running with a thou or two or play with so ill-effects. Any extra noise the engine might make as a result is masked by the mechanical cacophony from Vincent-HRD engines in general. I'd rather my valves were closing properly through the rev range than be too precious about Stevenage specs and end up burning a valve or wrecking a piston at high cruising speeds.

PK
 

derek

Well Known and Active Website User
VOC Member
hesitation

A gobule of water (rain or from the fuel etc) below the main jet gets sucked up at wider throttle openings and as the water won't pass through the jet so restricts the fuel passing through. This has happend to me many times over the years. Derek.
 

hooterman

Well Known and Active Website User
VOC Member
I have experienced this problem on my 52 shadow- no problems for weeks then a sudden dip of power at high speed. Talking to my old man, we think it must be a shortage of fuel as the mag works well (first or second kick to fire), many miles of problem-free riding, then an occassional lack of power. We have checked both fuel lines into the carbs and all seems well, but will continue to look at fuel starvation as the main cause.
 

stevew51

Website User
Non-VOC Member
Power loss problem

Thanks for the reply, I also think this is a fuel problem. With the bike on the rear stand I can bring it up to full speed with no problems. So it has to be under load for the problem to show up. Lack of fuel would definatly do this but I have been through both carbs from tank to exit flange and I can find nothing wrong, The mag might cause this power loss also, due to the higher compression in the cylinders. But why would a mag fail at a high speed when even a weak mag will jump a 3" arc. Possibly a short or a ground caused by centrifical force or a thrust bearing failure.
I've tried pressing down on the ticklers during these power losses to see the effect and it makes no difference. I've tried decreasing the spark plug gaps and it has made no difference, either. Compression on both cylinders, hot is 120#, which is good. Let me know of any break throughs w/ your problem.
 

Robert Watson

Well Known and Active Website User
VOC Member
Look for simple first

Check the vent in the gas cap. all fine at low RPM or revving with no load. Longer pull with load, starves for fuel and get very sluggish. Been there done that. Had a tank bag on so didn't see the obvious. Left Vancouver and was in NorCal when it suddenly started. took about 3 long runs before it was spotted. That what happens when you polish too much, and leave a bit in the cap vent!
 

Bracker1

Well Known and Active Website User
Non-VOC Member
Greetings again, It appears you've checked the possible fuel problems, mag and spark, as well as airflow and valve adjustments. Quite possibly your idler gear has slipped due to distortion of the woodruff key, recheck that all marks align. This would mess up the valve timing. Stranger things have happened. Keep us posted and good luck, Dan
 

stevew51

Website User
Non-VOC Member
Hesitation problem

Dan,
I checked the idler/cam gear marks and had a eureka moment when I found that the marks do not line up. They are all off 1 gear tooth when the two "B" marks line up to the mag. idle gear/fwd cam gear. I talked to a local Vincent specialist at Coventry Spares and he said the marks do not necessarily mean anything. (He only ref.s the valve's position to a degree plate.) Also, Stevens in "Know Thy Beast" states that the idle gear can be off one gear tooth. So I'll have to check the valve's w/ a dial indicator to throughly check the engines timing. The gears are in good shape, tight and no damage can be seen. So I am back to thinking it may be a mag problem, due to spark failure on higher compression. Thanks for the advise. Steve
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Timing marks

It isn't unusual for Comet marks to be off, but I've never heard of a twin's marks being off. Which doesn't mean it hasn't happened - but mine wasn't one of them if it has.
There's a comparatively easy way to find out, true for all engines I've ever worked on. It has the advantage of sidestepping variations of - and endless arguments concerning - tappet clearance, whether to time on opening, or closing, or...
The valve lift should be equal at TDC +/- 5 degrees, preferably on the early side.
Is that it?
Yup.
This has proved to be true for my Vin Twin, a Manx Norton, a Rudge Ulster, and a Honda 350 K4 racer I once had. The Honda was interesting since it had an HRC camshaft (i.e. a factory race kit part) and I timed it to the usual IO / EC figures. Then I checked. Both valves had equal lift at 3 deg BTDC.
Hoping that I'd just made a discovery of earth-shattering importance, I 'phoned the guy who builds my Manx motor to ask him about it. He was depressingly matter-of-fact. If the valves of a Manx have equal lift between 5 BTDC and TDC, it'll run perfectly. Bugger. There's my Nobel Prize down the tubes.
I actually timed the Ulster to equal lift at TDC in the absence of any other figures, and it flies, pulling perfectly throughout the rev range.
What would save a lot of time, and cost about £45, is to find someone who knows what they are doing, who has a dyno with an exhaust sniffer. If you Google "highland dynos" you can see what can be done. Since this guy is in Cromarty I don't expect to be accused of advertising, but I'd try to find someone who does race bikes, because he wouldn't survive if he wasn't good. Something that might be good to know: when the club shadow was dyno'd, there was very little difference between the sniffer results on individual pots, and on both together.
 

stevew51

Website User
Non-VOC Member
Reply

I pulled the mag off and sent it back to the guy who rebuilt it last fall. I've looked into dynos also and I have found one close by me. (Cape Cod, Massachusetts). Hopefully the mag will be the problem and I can do some riding this summer. Thanks for the feedback, Steve
 

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