First run of the year, Problem!

I had a fuel problem when I took my old MG out for MOT after being laid up over winter a few years back- it cut out and died in the middle of the countryside. Out of gear I could get it to tick over and rev somewhat but it wouldn't take any load.

I tried a few times of holding the revs high while stuffing an old blanket over the carb air intakes, until the engine stalled (ie. using the full force of the induction stroke to drag fuel through the jets in the absence of air to drag in across the jets).

After a couple of trys it started revving freely and I was on my way..
 
Piet ( good call ) is pointing out one of the most simple things to check,  yet gives a great deal of trouble if not set to correct height, accurately it needs to be checked. Use a small fibreglass L shaped "square" to set mine.  Way more easy to check than reason out all the issues incorrect height causes,  experiences of  the road worn... Never been able to run a triple on 145 mains ever in 17 years out here in Australia.  132-138 mains maxiumum.  With decent ignition you can run 55 pilots.

7300rpm in top..... you are the man!

HTH John
 
When i bought my Jota it wouldn't pull the skin off a rice pudding. There were several reasons for that, one of them was 135 mains (Dell carbs). Way oversized. It was running an A/F ratio of around 11:1 at WOT. Went down to 130, 125, 120, Checking A/F ratio on the dyno. Got somewhere near 14:1 with 120 jets. Gain of around 5 RWHP and much cleaner throttle response when opening it up.

Oddly enough, going from 135 to 130 made very little difference - identical A/F trace on the dyno chart. I guess it couldn't flow enough fuel through the needle jet to feed the 135. It probably would have run just the same without any main jets in it at all. This has me wondering whether anyone with bigger main jets than that 130 in Dellorto carbs is actually getting increased flow through the jet orifice.

After a lot of farting around on the dyno, I've zeroed in on carb settings that are almost identical to factory specs (only difference is 120 mains rather than 118). Seems someone knew what they were doing in the factory.

Cheers,
Cam
 
Dellortoman said:
Seems someone knew what they were doing in the factory.
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As is quite often the case... at least concerning the Breganze Beasties! :D :D

Different motor Cam, but 2-1/C5 or C6 equipped 750 SFCs go as high as 160-170 mains, with the same 265AB needle jet as the triples.  Don't remember the specified needle, but the difference won't be THAT big.  The long valve-timing overlap probably reduces intake vacuum drastically, the 2-1 not helping much either in that case, hence the monster sized jets.

piet
 
The likely reason it dies in 5th is it finally runs out of fuel. The floats are high enough to keep it running long enough to get to 5th but with the demand at WOT pulling longer the fuel flow can't keep up. Had the same problem on my race bike,drove me crazy till I figured it out.
 
If the float height has gone out why does it rev out in 4th and looses power @ 7.3K in 5th?

May be it is the new fuel taps I fitted, I checked the bore of the original taps and they were 5mm, new ones are slightly less.

Wll check float heights first then replace taps again if I can find some high flow types.
 
Most bikes have only one fuel tap and manage just fine to flow enough juice.  The Laverda has two.  Reckon your problem is elsewhere.

My 180 went like a train up to 7500, but wouldn't pull any more in top.  Head needed some work, I reckon.
 
I would think 7300 in top is what they all do as standard (and old and used).
Are you sure it pulled through this before all the changes you made?
 
I've checked the float bows previouslt and tghey were clean.

The point is that it will rev to 8k in 4th without any problem but wil only go to 7.3K in top.  The engine goes flat as if its not getting enouigh fuel, but why only in 5th.

Anyway I'll remove the carbs tomorrow evening and check the floats etc then the only thing left are the new taps!

Ho hum.
 
Bob, it's to do with the gearing.
To be able to pull top revs in top gear is not a given.

Did it do it before the changes?

If you really need it to pull top revs in top gear fit a bigger rear sprocket.
 
RCGBOB44 said:
The point is that it will rev to 8k in 4th without any problem but wil only go to 7.3K in top.  The engine goes flat as if its not getting enouigh fuel, but why only in 5th.
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Because the load on the engine is a lot higher in 5th, simple!

Too high float levels will cause fuel starvation, high-flow taps or not.

As Grant has pointed out, stock taps are more than large enough to guarantee ample fuel supply.

Float height should be 18-19mm measured from bowl sealing surface.

piet

 
Piet

I'll check the floats tomorrow.

Grant

The problem is that the engine is dying at 7.3K in top gear under load and not in 4th.  It's not like an engine that just won't pull top gear because of gearing, it's actually changing note and dying untill you shut off for a moment thewn open up again at which point the engine chimes in strong then dies again and looses power.

will report back tomorrow on the float height check.
 
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