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inconsistent running on highway - Surging and failing

Posted: Thu Dec 29, 2011 7:18 pm
by Gavin Dufty
Gidday , any help please perplexing problem -

i have rebuilt a 750 sport 1975 engine (roundcase) - and have installed dyna ignition

i was originally running 3 amp coils and it kept toasting the ignition system this would happen within 50 km of start up (yes i did three of them) OUCH$$$

- i call dyna and they said drop the coils back to 5 amp (which i have) and it seems to not be okay i have done over 100 km now and no blown fuses and toasted ignition

but the bike still surges and fades when on the open road and at consistent throttle opening - its fine around town ( but then again i dont hold throttle at consistent opening ) - yes its been tuned

its driving me spare

any suggestions would be great

happy new year to you all

Posted: Fri Dec 30, 2011 4:27 am
by abmartin
Are you certain its an ignition problem and not carburation?

Bruce

Posted: Fri Dec 30, 2011 2:35 pm
by Crashfangio
I agree, sounds like fuel starvation.

Posted: Fri Dec 30, 2011 7:27 pm
by wdietz186
As Bruce and Crashfangio have mentioned it could be carb related. The first thing I'd be checking after the flow from the petcocks is the float levels. If too low the bike will run out of fuel under a constant fairly high load. With the carbs inlet down and on a level surface and the float tab resting on the float needle with the valve closed but not compressing the light spring in the float valve the bottom of the float should be 17 mm from the floatbowl sealing surface. It is easy to overlook after a carb rebuild if you have changed the float valves or replaced the floats. Another thing to check is the screen behind the fuel banjo where the fuel line attaches.The fine mesh catches lots of crap.

Posted: Sat Jan 07, 2012 8:44 am
by Gavin Dufty
Thanks for the replies i think it is fuel flow issue - blocked filters in fuel taps

kind regards

Gavin

Posted: Thu Jan 12, 2012 12:55 pm
by MitchB
That could be it.

Don't forget to look at the filter screens on the Del Ortos (at the fuel inlet), Make sure the gas tank is venting correctly (poor venting - gasping - caused the exact symptom on another bike of mine), and the intake manifold gaskets.

All easy to check, cheap to correct.

I am finishing my bevel project and have swapped from stock to 5 ohms Dynas. Did they improve performance for you?

Posted: Fri Jan 13, 2012 4:02 pm
by Gavin Dufty
Thank Gents i think i have it sorted - simple - vertical cylinder fuel line to carbi - was pinched and restricting fuel flow ( so now new petcocks, fuel line, seem to be all good - may be running a bit rich now ) - the joys of bevel drives - always seems to be a reason to tinker with them - i suppose thats half the fun.

Posted: Mon Jan 16, 2012 1:17 pm
by Crashfangio
What size carbs, main and pilot jets are you using??

Re: inconsistent running on highway - Surging and failing

Posted: Fri Jan 20, 2012 4:58 pm
by Gavin Dufty
that is a really good question i am unsure of cabi jet sizes - the cabs were totally rebuilt (along with the engine) by motocraft - i will call and find out
Gavin

Re: inconsistent running on highway - Surging and failing

Posted: Sun May 13, 2012 4:30 am
by My750GT
Interesting thread here. I have been having the exact same problem with my 72'. Also on the vertical cylinder with Dyna S. I assumed it was the ignition because I didn't have the correct tools and gadgets when I set it up. A little tuning by ear improved the issue but it was still there at cruising 60/80kmh. No kinks in lines or blocked filters but I will be looking at the pet-cock as it probably hasn't been looked at in 30 years. To think I was the only guy to have issues with an old Italian motorcycle :lol: :lol: :lol:

Re: inconsistent running on highway - Surging and failing

Posted: Sun May 13, 2012 2:00 pm
by Bern
Also look at the vent on the gas cap. There was a home made gasket on mine that had no vent hole in it, so there was a bit of a vacuum in the tank when running for prolonged periods on the highway. This happened on mine and I opened the gas cap and you could hear the air rush in.

Re: inconsistent running on highway - Surging and failing

Posted: Fri Aug 02, 2013 1:33 pm
by thessler3
My '73 750GT is surging a bit too. It feels like the rear cylinder is misfiring just a bit when cruising along at light loads around 3000 to 4000 RPM. It runs clean and strong when accelerating. The rear cylinder exhaust pipe is much darker than the front - appears to be running richer. I have a Dyna ignition newly installed and old AMAL carbs that I have cleaned 3 times since my gas tank was full of varnish and had to be cleaned. Both the tank and carbs are clean now. I did adjust the rear Dyna pickup to set rear cylinder static timing to 10 degrees bTDC, same as the front cylinder. I think my rear carb has a slightly damaged needle jet and I am going to replace all the jets, the needles and the slides if I can find them. Amals are simple, but not particularly durable. Also waiting for new throttle cables before carefully syncing the carbs. That could also be why the rear cylinder is running richer. If anyone has any ideas, I would like to hear them.

Re: inconsistent running on highway - Surging and failing

Posted: Mon Aug 19, 2013 5:20 pm
by thessler3
running good now with new needles & jets in the AMALS. Set needles to top notch to cure surging. I am using 100LL aviation gas. Those using pump gas may need different settings. I did run it briefly with 10% ethanol pump gas and it ran well - even better than with the 100LL aviation gas. Had to drain it quickly though - since I have a fiberglass tank.