WEIRD CARBURATION FAULT

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simonnorthroad
Posts: 388
Joined: 16 May 2017 10:57
Location: Bath, UK

WEIRD CARBURATION FAULT

Post by simonnorthroad »

Ive got a strange carburation issue on a 350 ive just bought. The bike starts, and ticks over, both cylinders getting hot, though one seems hotter than the other. Compression is good on both cylinders (165psi) but the rear carb doesnt seem to be working properly.
Screwing the mixture screw out or in has no effect on the rear cylinder tickover until the last fraction of a turn from right-in, when it sounds like it finally 'chimes in' and the tickover climbs. Ditto screwing in the slide screw, only the last fraction of a turn has any effect on the tickover, when it rises.
Ive had the carbs ultrasonically cleaned TWICE by a scrupulous engineer who checked all the passageways and blew them through.
The bike starts with no chokes (a first on a Morini for me) and even though I replaced the 50 pilots with 45s it still does exactly the same, no choke and wont start with the chokes. Ive replaced the flip-up chokes with new which made no difference. The starter-jet O ring is in good condition. Im a bit flummoxed tbh as its clearly some kind of carburtion fault just with the one carb
morini_tom
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Joined: 05 May 2006 13:47
Location: Northampton

Re: WEIRD CARBURATION FAULT

Post by morini_tom »

Have you checked the float heights?

And perhaps more importantly, have you checked that the floats don’t leak? Weigh them down in a container of water (I wouldn’t dare suggest using a Tupperware or saucepan from the kitchen) and leave for a good period to see if they take on any water.
MickeyMoto
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Location: Even further oop North

Re: WEIRD CARBURATION FAULT

Post by MickeyMoto »

Manifold leak?
3potjohn
Posts: 1243
Joined: 02 Jun 2007 13:58
Location: Devon

Re: WEIRD CARBURATION FAULT

Post by 3potjohn »

On that note is it possible to swap the floats and float bowls and see if the fault moves across.It sounds like too rich to run. What happens if you have the fuel off -will it pick up at all before running out?
John
simonnorthroad
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Joined: 16 May 2017 10:57
Location: Bath, UK

Re: WEIRD CARBURATION FAULT

Post by simonnorthroad »

Yes, the float-heights have been checked against the book and absolutely spot on - good try though, thank you! No leaks.

Ive sprayed carb cleaner around the metal manifolds and stubs which seem tight and the revs dont rise so Im PRESUMING no airleaks
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Daddy Dom
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Joined: 16 Nov 2006 19:48
Location: Auckland, New Zealand

Re: WEIRD CARBURATION FAULT

Post by Daddy Dom »

I can offer a few basic visual detective-work suggestions:

If you remove the mixture screws and springs, are the screws identical? Swap them over and see if you get the same thing.
How about removing the enrichers and swapping them over too?
Are both throttle-cables the same length? Are they both sitting correctly in their adjusters?
Are both carb slides identical and identical heights? Are both slides the right way around?
Swap and clean the plugs too and check the gaps because 50% of all carburettor problems are spark-related ;)
Are both plug caps the same? Not one with a resistor and the other without?

My 2½c worth.
DD
MRC 3082½
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72degrees
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Location: West Midlands

Re: WEIRD CARBURATION FAULT

Post by 72degrees »

I had a similar issue with the carb on my 1970 Gilera 98. A Dellorto ME18BS in this case. Ridiculously rich at idle. The mixture screw had been broken off - I guess by someone vainly trying to screw it in far enough to lean it off a bit. I acquired another used ME18BS off Ebay and swapped the jets to create a combination that seemed about right according to the manual. No better. So rich that black smoke was billowing out at tickover and the plug was sooting up. I even tried a NOS float (height is not adjustable on these but the needle looked a bit stepped.

In the end, I looked very carefully at the jets again. On comparing the atomisers, which were nominally the same (258A), I could see that the hole in the top of one (from the bad carb) was much bigger. The slightly newer looking one had a reduced diameter just below the tip. The other one was either very, very worn, or some Italian youth (the bike was first registered in Bergamo) had drilled it out in a misguided attempt to enhance the claimed 6HP. The gap between the slide needle and the jet must have been so big that mixture was being sucked through that way rather than via the idle jet and circuit. With the atomiser changed to the unmolested one, normal starting and service has been resumed - though mid range also needed tweaking a tad by a change of needle number and position.

No doubt you have checked that both carbs have the same (correct) type of atomiser, but it might be worth giving them a very close inspection with a magnifying glass.

If it has an electric fuel tap, experiments to see if it revs up as it runs out of fuel are a bit harder to achieve, but if you can get it running on just the rear pot (front HT leadoff) and deny the rear carb fuel supply, a rapid increase in revs as it runs out would be sure sign of a very rich condition at idle. My 98 would rev so hard as it was running dry before I sorted it out, that I had to quickly kill the ignition.
3potjohn
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Joined: 02 Jun 2007 13:58
Location: Devon

Re: WEIRD CARBURATION FAULT

Post by 3potjohn »

I had mismatched jets when i got my bike-this may have been a tuning mod I suppose but I did not spot the different numbers for a while. I rebuild the carbs shortly thereafter. Only time I have ever had a running rich situation was on my BMW r60/7 which has choke pistons with a "neoprene" tip.These break up every 10-20 years or so causing a choke effect. Progressive reduction in need for any choke from cold. I have recently replaced the pilot jets on this bike and now it needs choke for a couple of hundred yards so I guess the old ones were worn or possibly there is some variation in spec.

John
simonnorthroad
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Joined: 16 May 2017 10:57
Location: Bath, UK

Re: WEIRD CARBURATION FAULT

Post by simonnorthroad »

Cheers chaps. All good advice. Id taken the electric-tap off, as it wasnt passing much/any fuel through. Will have a play with your suggestions and espec the atomiser suggestion. Let's hope its not one of those now unavailable sizes
simonnorthroad
Posts: 388
Joined: 16 May 2017 10:57
Location: Bath, UK

Re: WEIRD CARBURATION FAULT

Post by simonnorthroad »

Checked the atomisers and they are the same, 250Ks. Tried a pair of 250BDs, but as Eurocarb here in the UK warned me, they dont affect tickover... so not a jot of difference

Eurocarb also told me to check for an O ring and washer on these early VHB mixture screws, but as the cylinder only chimes in with the mixture-screw almost fuklly in, Id have thought these extra packing-parts would make it worse...
texaskitty
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Joined: 17 Jan 2016 06:37
Location: Melbourne, Australia

Re: WEIRD CARBURATION FAULT

Post by texaskitty »

On both heads of my '75 350, the flanges of the metal inlet manifolds have cupped slightly. If I ever get this thing back together I must remember to carefully sand the flange so that it's flat/true (on some plate glass perhaps) and/or use some sealant between the flange, gasket and 'antithermic flange' (the thick black thing). I am not sure that there is enough give in the 'antithermic flange' to make up for the cupping on the inlet manifold.
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