Motor slows down after a minute at WOT

dman535 · Post by **dman535** » Wed Jun 10, 2009 4:34 am

My Forty Plus starts up great - runs well. Except that when I go wide open I get about a minute or so of full power then the motors backs off a bit, even surges a bit - which leans toward a fuel issue in my mind.

After I throttle down for a minute or two - the motors runs WOT again no problem. Then the cycle repeats.

Any ideas on what to check or look at? I cleaned out the carb - motor doesn't seem to have fue issues - but I am no expert. Is there a tank filter I should check?

Any other ideas?

Thanks

Derek.-

Waggles · Post by **Waggles** » Wed Jun 10, 2009 6:56 am

Hi Derek,

If you could post the engine number the real experts could probably be more specific.

Yes, it does sound like fuel starvation ( mixture going weak as the fuel supply can't keep up with the demand at WOT ) Assuming you have the most common villiers carbed 40+ there most certainly is a tank filter to check, its on the top of the fuel tap, just remove the tap, there is also another in the 'banjo' connection where the fuel line connects to the carb. Check also you don't have an air lock / bubble / sediment / goo in the fuel line.

There are other things that could cause this ( like an air leak somewhere that only opens up fully when hot - motors can get very cranky when they have air leaks ) but I would check the filters first.

dman535 · Post by **dman535** » Wed Jun 10, 2009 12:51 pm

The number is : FP 2311-bb6

I have had a few other posts dealing with a crank-case leak. I did an external seal with some high temp rtv silicone. Its stopped the leak - and the motor seems to run better than it did before. Its still got this WOT problem.

Thanks

Derek.-

Keith.P · Post by **Keith.P** » Wed Jun 10, 2009 3:13 pm

If there are no more leaks on the crank case, then it must be getting more air in from somewhere, have you got any play in the crank itself, as they are only bushed and they maybe a bit worn and could pull air in at WOT, other than that pop the carb needle up one notch for a stronger mix and give it a run.

Vic · Post by **Vic** » Wed Jun 10, 2009 3:50 pm

If it starts OK, runs Ok at part throttle and initially at WOT then I'd go with the blocked filter theory.
Either the one in the tank which most people know about or the one on the carb inlet that they dont.

Also be sure that the tank is venting properly

dman535 · Post by **dman535** » Wed Jun 10, 2009 3:59 pm

Am I going to need any parts/seals/washers etc if I start taking the fuel line apart?

niander · Post by **niander** » Wed Jun 10, 2009 5:17 pm

no just go ahead and investigate
there is a small hole in filler cap ...must be clear...or vacuum...no fuel allowed out!
take off filler cap does problem go away?

Hugz · Post by **Hugz** » Thu Jun 11, 2009 10:52 am

Have to agree that it sounds like a filler cap venting problem. Try running her without the cap (flat water!). If not, jiggle the jiggler to see if there is plenty of fuel in bowl when the revs drop.

Collector Inspector · Thu Jun 11, 2009 11:25 am

I love Jiggling my jiggler.

OK. Sorry, all the above is for fuel except you have not specified oil mixture that it is on?

If all above is cleaned and jiggled and correct mix with little or no improvement, open the gap on your plug a little bit.

Kinda "Little Jiggle" like..........0.025" Start at 0.028" first and work back slightly.

Never know, could be "Juicing Up" like the yanks say.

Another thing to try is WOT and then back off till the revs drop a tad. You will hear a slight drop in revs and see how that goes. Gets the carb back into "Perspective"

All Old Dears are different. Like a H.I.D, takes a bit of practice to work them out individually.

Best of luck as I have had a few HIDs and I failed to work any of them out...........

Regards

C.I.

dman535 · Post by **dman535** » Thu Jun 11, 2009 6:02 pm

The motor is a 10:1. I am going to dig into the carb tonight and verify the position of the needle and check the fuel filter.

Does a change to a 25:1 mixtures ( with the different needle) have any impact on engine performance ?

Post by **charlesp** » Thu Jun 11, 2009 6:08 pm

If you change from 10:1 to 25:1 there would theoretically be no performance changes. The reason that changes are made to the needle or jets is to adjust for the lesser viscosity of the lower oil content. This prevents the motor from running too rich.

There is a school of thought that would suggest making noi change in your case until your problem is sorted. If it should turn out, as has been suggested, that your bearings are a little worn, allowing air into the crankcase, then less oil may make the problem worse.

In general I prefer to leave the motors at 10:1 if that's the way they were designed.

dman535 · Post by **dman535** » Thu Jun 18, 2009 4:34 pm

I cleaned out both filters - they didn't look to bad.

I also adjusted the depth of the jet, flush + one turn in. Made a big difference in how the engine ran at top end. Might need yet another half turn - but I think the issue is resolved.

Thanks to all who replied.

Derek.-