Problem with 750 GT

Well I ended up double checking the valves and re-checked the gaps, inlet at .15 and the exhaust at .20.

I started her after that and she seemed to idle reasonably well for about 5-10mins then all of sudden she started missing and backfiring and stopping. I left her for 5 mins and then restarted her and she settled down into a good idle and I couldn't hear her miss as before, so let her idle a while longer and tried elevating the revs to 3000rpm and hold her there for a period and all seemed good no loss of power or backfiring....I even put the strobe on her and it seemed good with both the numbers appearing 1 & 2 at the same time and advancing with revs.

So I turned her off and put her back together but when I restarted her she wasn't running as well, this still with the DMC on the 10th curve that I was told to put her on and have been using after using the number 1 position on the DMC.

She started to miss again....this when I decided I have had enough and turned her off and packed up for dinner...the brain is tired.

I am in the process of uploading a youtube video of when I videoed the bike running. when done I will attach the link for you to see and make comment.

Thanks

Brett
 
Go on.

Swap the coils over and see if the problem moves with it.

H.
 
Hi Brett,
Was there  a problem with how the bike was running before fitting the DMC?

Could you have a fuelling problem ,dirt from tank finding its way to carbs?

Andy
 
Dellortoman said:
So where is the ignition timing taken from?

In future I'll refrain from commenting on 750s. Not having seen inside one ...
Click to expand...

The ignition is driven at 1/2 engine speed fom the oil pump shaft, but the DMC rotor has 2 gaps giving a spark every 180? of oil pump shaft rotation, or every 360? of crank rotation.

piet
 
Well Harry I hate to disappoint but swapping the coils made no difference she still just drops power, I had her revving at 2000rpm and then suddenly no power and off.

I am really starting to think maybe I have a dodgy DMC2 as I have tried everything and still have the same issue just the missing and drop of power.
 
Brett64 said:
I am really starting to think maybe I have a dodgy DMC2 as I have tried everything and still have the same issue just the missing and drop of power.
Click to expand...

It does happen Brett, I had a dodgy one on my SFC 1000. (I should add that Uwe Witt and Wolfgang were excellent in their assistance to rectify the problem) :D
Any chance of swapping the box with someone else's for a quick try?

Cheers
BK
 
Hi Bruce,

Not where I am, I have sent an email to Greg Parish for assistance as I bought it through him a few weeks back, I know he has a few personal issues at present but he asked me to send an email and that he will get to it when he gets sorted, I told him that there was no rush just get himself in order with his kids first.

I could give Geoff A as he lives down Laurieton way and may have some knowledge having a couple of SF 750 Laverda's and also re-built a few motors.

Anyway I have a few other bikes to ride in the meantime and will have to commission the 1928 Indian to get it running when it arrives in another two weeks as it hasn't been started since its restoration some 3 years ago. That maybe another fun challenge!!

Cheers

Brett
 
Just re reading the DMC instructions.

Under Test Run item 4

The DMC has a "Idle regulation system" so at idle speed 1000rpm the advance switches between 0 and 8 deg BTDC with increasing revs the system should go to full advance ~32 deg (depending on curve) at about 3000 RPM

Jugs_S
 
Yes I just tried all of the different curves on the off chance the advance might not have been right at curve #10, but to no avail it is still missing intermittently and losing power.

I re-checked all of the wiring to ensure no poor connections but nothing found.

Anyway Greg is going to send me up another DMC2 box to test to see if this one has a fault.
 
I had the same problem with my DMC on my SF1. I was using resistor plugs, but there was still too much electrical interference which raised cain in the brain.

My solution was to 1) ditch the resistor plugs, 2) use NGK 5kOhm plug caps, and 3) move the ignition as far toward the rear of the bike as possible. I mounted it on foam tape on the rear fender - I know some people put it in the tool box. I would have done that, but my toolbox crumbled into fine Italian pumice years ago. Just wrap the ignition module in foam or a rag and stick it in the toolbox.

After doing 1,2, & 3, my bike immediately ran like a top, and has continued unabated ever since.
 
Grunbay said:
I had the same problem with my DMC on my SF1. I was using resistor plugs, but there was still too much electrical interference which raised cain in the brain.

My solution was to 1) ditch the resistor plugs, 2) use NGK 5kOhm plug caps, and 3) move the ignition as far toward the rear of the bike as possible. I mounted it on foam tape on the rear fender - I know some people put it in the tool box. I would have done that, but my toolbox crumbled into fine Italian pumice years ago. Just wrap the ignition module in foam or a rag and stick it in the toolbox.

After doing 1,2, & 3, my bike immediately ran like a top, and has continued unabated ever since.
Click to expand...

WRONG....

The DMC unit is grateful for a bit of cooling air.  No big problem in moderate climates, but once ambient temps start climbing above 35?C, the box will add to that with its' own generated heat.

Really cannot understand people not sticking to installation instructions, they're usually the first to blame the component when things go tits up.  Manuals are not written because some smartarse had a day off with nothing else to do...

piet
 
Have not yet seen a Haynes manual covering DMCs... :D  Agree, many of those are not worth the paper they're printed on.

Not too difficult to follow most ORIGINAL installation instructions.  Lots of thought goes into most of them, things turn sour when they're incorrectly translated from Mandarin chinese to french, then to zwaheli until finally into english, bound to have things drop off by then.

But why ignore the rubber mounts and chuck a rather costly bit of equipement into a rag-stuffed toolbox and hope for the best?  I've seen it several times, a couple of those owners complained over intermittent misfires and eventually stalling, only to have the bike running again after a cooling pause.  A mate of mine (who should know better, he was involved with the initial development of the DMC I) was just as lazy, been running his SF3 for the past 20 years with the DMC nicely wrapped in a blanket.  It finally cried enough a couple of years ago while on a vacation trip through sunny Italy.  Temps hit 40?C and the bike stopped.  After the unit was checked by Mr.Witt hisself upon returning, it was deemed fried, my mate was very lucky to get home under his own steam.

piet
 
C'mon, Grunbay, give Piet a go.

When you read trough this forum a bit, you might learn that he has quite some real-life personal contact to Mr. Witt, who is the developer of the DMC unit. By reading this topic alone, you might even get a feeling which tremendous amounts of time Mr. Witt probably has spent on resolving RTFM cases during the last couple of decades. >:(

Taken from that point of view, Piets comment looks just well tempered to me. 8)

...and yes, I've seen myself those units,  mounted a top a stinking hot triple engine, nicely paired up with the coils :-X
 
Hi Piet,

Yes I have it mounted in the toolbox correctly as per the instructions and as per Greg Parish informed me, the only pain is to access the curve adjuster you have to unmount the toolbox and the DMC box, but not a major problem.

Greg has sent up a replacement DMC box for me to test which I will do tomorrow when I collect it from the Post Office.

I started it up this afternoon and it still had the same problem, it idles but misses intermittently and losses power and stops running occasionally, at low revs idling there is a knocking noise coming from the clutch, I have checked the primary chain and seems good so I am thinking it is the rubber cushes so I have some new ones on the way.

Cheers

Brett
 
Hi Piet,

Well the DMC part arrived and I tried it out today but still the same problem, she starts but misses intermittently and will stop running but will restart easily.

I checked the circuit board in the headlight while she was running by wiggling all of the wires but no issues.

Could it possibly be the regulator? It is the original one.

Could this cause the intermittent missing?

I put a new battery in the other day but I need to recharge it on trickle after numerous starts.

I tried swapping the coils but nothing changed and I let it idle on each cylinder separately but both ceased running intermittently again so it can?t be the coils.

Regarding the knocking it is definitely coming from the clutch location as you can feel it through the primary cover right adjacent to the clutch hub area. I will take the primary cover off again to check the clutch.

Any ideas??

Cheers

Brett
 
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