inconsistent running on highway - Surging and failing

If you need technical information or help with your roundcase Ducati 750 engine - post your FAQs, comments & questions here.
Post Reply
Gavin Dufty
Cucciolo - the Lil Pup
Posts: 5
Joined: Thu Dec 29, 2011 7:00 pm
Location: Australia

inconsistent running on highway - Surging and failing

Post by Gavin Dufty » Thu Dec 29, 2011 7:18 pm

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

User avatar
abmartin
SSD900 Darmah
Posts: 358
Joined: Tue Apr 10, 2007 4:08 pm
Location: New Brunswick, Canada

Post by abmartin » Fri Dec 30, 2011 4:27 am

Are you certain its an ignition problem and not carburation?

Bruce
1979 900SS
Fredericton, NB
Canada

Crashfangio
Mariana
Posts: 35
Joined: Fri Aug 15, 2008 5:48 pm
Location: Melbourne Australia

Post by Crashfangio » Fri Dec 30, 2011 2:35 pm

I agree, sounds like fuel starvation.
73 750GT

wdietz186
888
Posts: 653
Joined: Sun Sep 10, 2006 4:40 pm

Post by wdietz186 » Fri Dec 30, 2011 7:27 pm

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.

Gavin Dufty
Cucciolo - the Lil Pup
Posts: 5
Joined: Thu Dec 29, 2011 7:00 pm
Location: Australia

Post by Gavin Dufty » Sat Jan 07, 2012 8:44 am

Thanks for the replies i think it is fuel flow issue - blocked filters in fuel taps

kind regards

Gavin

MitchB
Mariana
Posts: 35
Joined: Wed Sep 09, 2009 4:49 pm

Post by MitchB » Thu Jan 12, 2012 12:55 pm

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?

Gavin Dufty
Cucciolo - the Lil Pup
Posts: 5
Joined: Thu Dec 29, 2011 7:00 pm
Location: Australia

Post by Gavin Dufty » Fri Jan 13, 2012 4:02 pm

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.

Crashfangio
Mariana
Posts: 35
Joined: Fri Aug 15, 2008 5:48 pm
Location: Melbourne Australia

Post by Crashfangio » Mon Jan 16, 2012 1:17 pm

What size carbs, main and pilot jets are you using??
73 750GT

Gavin Dufty
Cucciolo - the Lil Pup
Posts: 5
Joined: Thu Dec 29, 2011 7:00 pm
Location: Australia

Re: inconsistent running on highway - Surging and failing

Post by Gavin Dufty » Fri Jan 20, 2012 4:58 pm

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

My750GT
Cucciolo - the Lil Pup
Posts: 24
Joined: Wed Feb 29, 2012 2:26 am

Re: inconsistent running on highway - Surging and failing

Post by My750GT » Sun May 13, 2012 3:30 am

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:
'72ish 750 GT
Functional bitser
Western Australia

Bern
Parallel Twin
Posts: 100
Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: inconsistent running on highway - Surging and failing

Post by Bern » Sun May 13, 2012 1:00 pm

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.

thessler3
Mariana
Posts: 35
Joined: Tue Jul 30, 2013 1:57 pm

Re: inconsistent running on highway - Surging and failing

Post by thessler3 » Fri Aug 02, 2013 12:33 pm

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.

thessler3
Mariana
Posts: 35
Joined: Tue Jul 30, 2013 1:57 pm

Re: inconsistent running on highway - Surging and failing

Post by thessler3 » Mon Aug 19, 2013 4:20 pm

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.

Post Reply

Return to “Roundcase Engines > 750 GT, 750 Sport & 750 Super Sport”