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General Category => General Discussion => Topic started by: Canuck750 on August 16, 2022, 10:54:02 PM
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I have a question for the brain trust, my 1973 Laverda 750 SF1 has been tuned by the go to guy in North America and it has run beautifully but recently it has this weird low speed stumble.
I took it for a 100 mile run today, mostly highway speeds, sustained 70 ~ 80 mph, minimal city roads at beginning and end of ride. At speed the bike runs super, pulls real nice, ran it up to just under 100 mph and there is more to go. On the way back into town stopping at a light when I pull away it feels like it cuts in and out a couple times until it gets going then its fine, next light same thing. This did not happen at the beginning before the bike had a good warm up and ran at speed.
The bike has electronic ignition and brand new Dellorto PHF 36 carbs, jetted by the expert for this bike.
Pulled the plugs this evening, one side just a little bit leaner than the other but both show a lean burn to my eye.
(https://i.postimg.cc/QNmvJjzy/D8215-A65-E7-D4-4-BBF-913-F-4-A3-CB54-C8-E95.jpg) (https://postimg.cc/WtD92cTG)
I could swear it feels like the ignition is switching on and off but I am beginning to wonder if I have the bike running too lean on the idle circuit and once very warm on the start off its starving for fuel?
Whatcha think ?
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Could be a coil acting up when it gets hot? Switch 'em from one side to the other?
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second the coil suggestion.
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After a good run like that I'd check the intakes for tightness. Possible vacuum leak at the boots or gaskets. My $.02
Paul B :boozing:
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After a good run like that I'd check the intakes for tightness. Possible vacuum leak at the boots or gaskets. My $.02
Paul B :boozing:
Good idea!
Wolfgang got back to me, he had installed new coils and high tension leads in the bike last year. He suspects a partial plugged pilot jet and he says Laverda like Champion N3C plugs better than NGK
I will check the keys this week and the carb boots as well.
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Vibration "unadjusted" the carburetors? I've seen it happen.
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I had same symptoms on 2 cylinder Honda and it was a bad sparkplug boot.
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Naturally, I normally say carburation problems are generally ignition :smiley: but that sounds lean to me. <shrug>
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Maybe try raising the needles a notch? DonG
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Maybe try raising the needles a notch? DonG
Thanks Don
I will give that a try, the pilot jets are spotless, I put in a pair of Champion N3C plugs as well
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Remember to only make one change at a time so as to be able to tell what actually cured the fault. DonG
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With the plug change to Champion N3C its better but once again it shows up after bike has had a good run down the highway and then has a few burps taking off the line again. Enriching the mixture screw a bit judging by the colours of the two stainless headers, and the plugs, failing any significant change I will raise the needles one notch.
Will see how that works on Saturday.
After my ride Thursday I noticed the rear signal lights were waging back back up and down, the Laverda shakes a 'bit', look what it did to the two year old rear plate and lamp bracket, both sides all split and deformed
(https://i.postimg.cc/fTJQyyGW/6-FB0-C6-DF-B699-4-DBB-B38-B-A0613-D878-BF0.jpg) (https://postimg.cc/7f8WQHPj)
a thick steel reinforcing plate should resolve that, thick washers spot welded to the exterior
(https://i.postimg.cc/BbZsMC72/44-FBB3-EA-B79-A-4670-ADA4-4-DA081881027.jpg) (https://postimg.cc/YGPJSQHS)
(https://i.postimg.cc/J7KRt9tx/AD52-EA86-B4-EA-47-AB-A70-F-0-C978-F1-C2839.jpg) (https://postimg.cc/bdG7BVxS)
(https://i.postimg.cc/zfWJBvRH/A5253-D26-7832-4-C13-AD07-54749-EB3-AA22.jpg) (https://postimg.cc/d7sMNq6J)
I am guessing the vibration will probably just be transferred to another weak point in the bracket
(https://i.postimg.cc/2yHYJBgM/88966-FB7-428-B-4692-B833-FC63-E4-B76905.jpg) (https://postimg.cc/1VNbV4mr)
I seem to be on a roll this summer, the Moto Morini 500 needs a new reg/rec as does my Moto Morini 3-1/2, the Eldorado is waiting for the left head to come back with the exhaust port threads repaired and the Laverda is keeping me guessing.
I get tired of old bikes sometimes....... :thewife:
(https://i.postimg.cc/xTyKDnR0/B28-EDCF4-2722-41-F7-A907-023-A5-B101-E5-A.jpg) (https://postimg.cc/6yq7CsgS)
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That assembly kinda looks like it came off of a Triumph or BSA.... :grin: DonG
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After another disappointing 2 hour run on Saturday, same abrupt stumbles on take off when warm, I stripped the carbs right down and repeatedly ran all the parts through a hot ultrasonic solution and blew the bodies out with compressed air, repeated this about a half dozen times until I got hem as clean as possible. The right carb in the clean ultrasonic solution did show a spiral of grey fine sludge coming out of the small ports that link the accelerator pump body. So there was definitely something still in there.
While I had the tank and carbs off I checked the spark plug leads to the coils and found the right hand side boot fitting had not been set all the way into a 'snap' fit.
Put it all back together, synced the carbs with mercury sticks, set the idle and took of for a couple hours. No more stumbles when warmed up on take off, bike runs smooth and pulls good at speed.
Now I don't know if there was a plugged idle circuit or if the loose high tension to coil was the culprit or a combination of both. After the run the plugs look similar in colour as do the colour of the stainless steel headers. At warm idle the header temperature about 2" off the head flange is about 165 F +/- a couple degrees left to right cylinder.
Just glad it fixed, old bikes, such a love hate relationship. I swore at the Laverda all last week and could have hugged it this afternoon.
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I swore at the Laverda all last week and could have hugged it this afternoon.
And there's the payoff. Great result. I knew you would get there!
I am in the closing stages of a 20 year quest to finish my 1964 XLCH. They test you!
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It could have been something in the carburetor but if it happened right off idle my suspicion is that it might have been a sync problem? Or possibly an air leak.
Regardless, glad you got it fixed and could enjoy it.
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And there's the payoff. Great result. I knew you would get there!
I am in the closing stages of a 20 year quest to finish my 1964 XLCH. They test you!
Twenty years! That’s commitment, like all your builds I bet it’s going to be a knockout.
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It could have been something in the carburetor but if it happened right off idle my suspicion is that it might have been a sync problem? Or possibly an air leak.
Regardless, glad you got it fixed and could enjoy it.
It may have been related to the sync. I hadn’t hadn’t checked the sync prior to stripping and resetting the fuel and idle screws.