Technical IMB28 on 594cc stalling issue

[createcartel]

Hi there,

I fitted a genuine new IMB28 on my ‘74 500R and despite it driving nicely for about 20mins, I’m having some problems with it.

If I open the throttle quickly, it chokes and dies. I’ve attached a video for reference. Ignore the loose exhaust, I sheared the bolts trying to swap it, so that’s another thing to sort!

Could it be getting too much fuel, causing it to flood the carb and choke when the throttle opens wide, like it does in the video? I haven’t changed the jets in the 28, so they’ll be 650cc spec. As it ran fine to begin with, I didn’t think it would be a problem.

What would you all suggest?

Thanks,
Matt.

 

[smart51]

More likely it is a bit lean. Snapping open the throttle lets more air in before the fuel can catch up.

What idle jet have you got? I've got a 28IMB on my 594. I swapped the 45 idle jet for a 50 which made it better. Try setting the idle mixture screw to 3 turns out and see if that improves it at all.

 

[createcartel]

Good question... it's a 28IMB 5/250 so I think it's going to be 0.50.

Just looking back at some photos, the engine is a 126A5 and it was previously running an IMB 24, not a 26.

I've got the exhaust off at the moment & need to drill the pesky sheared bolts out of the head Once the new exhaust is on, I can continue with the work on the carb.

I've changed so many elements to try to solve this (and also to give the engine a bit of much-needed love). I initially assumed it was an air leak, cleaned out the carb, refitted with a new gasket, etc. but then I went down the path of investigating the fuel side as I noticed my fuel filter was only about 1/8th full, regardless of whether it was fitted before or after the mechanical pump.

Right now, I've removed the pump and push rod, fitted a blanking plate, and an electric pump hooked up to a new fuel line, with a new pickup in the tank, fresh fuel, etc. With the filter immediately after the electric pump, it's still only got about an 1/8 in it — I can't remember if it was ever full, but it really does seem like it should have more fuel than just a dribble!

 

[smart51]

Yeah, mine's a 126A5 too. A previous owner had swapped the carb, but there was a 45 idle jet in there. I tried the usual procedure for setting the idle mixture but could never set it too lean. It ran OK at the maximum 3 turns out but when I put the bigger jet in there, it started much better. at 2.5 turns out, it would sometimes stall if dropping from high revs.

 

[createcartel]

Sorting the sheared exhaust studs went ok. I carefully drilled them out and tapped the M8 thread into the head. The main issue was that the engine mount was in the way, so I had to remove it. It took a bit of faffing about to get it all lined up, but it's all reassembled now — and with a new Monza exhaust.

I can get back on to the carb issue now

 

[createcartel]

Just circling back on this.

Some combination of fiddling got the 500 running well enough to drive it. I took it out in the evening and noticed I had issues with my lights, especially the indicator circuit. I assumed a bad earth, so in my quest to sort that, I found that my battery was showing 12.24v with the engine running (choke out to raise the revs).

The multimeter was also showing sporadic dipping in voltage, so I ordered an electronic voltage regulator, dynamo brushes and drive belt. I fitted the regulator and now I’m getting a much more respectable 13.4v to about 14v, but my dash light is flickering, so it looks like it does need the brushes.

Anyway, I fixed the lighting issue and took it out and enjoyed a 25 minute drive, at which point it started to lose power and feel like it was going to cut out. I pulled over and it did indeed cut out, only restarting after letting it sit and cool for a while. On my journey home, it died twice more.

I had just set up my rocker clearances (inlet 0.2mm, exhaust 0.25mm) so I’m wondering if the coil might be at fault. Anyone have any suggestions for a coil that doesn’t mind the heat?