15 replies to this topic

[whistler]

Been trying to sort an annoying habit my 1275 has of running rough  or lumpy when I'm slowing down or trying to crawl along at slow speed. I took a video today but can't upload due to file size so I've stripped out the audio and attached that. I'm going around a roundabout then accelerating away for about 300 yards then slowing for a turn left. Hope someone can offer me some suggestions. This annoyance has been with me for 18 months now. Changed Dizzy (65DM4), new coil, new cap, rotor and leads. Rebuilt HIF44, it's there with my 28/36 weber as well. Annoyance was there before I fitted S/Cuts.

 

Put this up on Facebook, here's a link to a video, I hope. https://www.facebook...14505825241222/

 

Edited by whistler, 13 August 2016 - 07:25 PM.

[whistler]

Now with video link.

[whistler]

Someone on a Facebook forum suggested geartrain backlash, so I rough measured the movement of front wheels when jacked up and in gear. There's a link somewhere I think from John Guess saying that 30 degrees is about maximum before a strip down. Mine is showing about 15 degrees. Another poster mentioned changing from BPR6ES to BR7ES plugs as his 1380 showed similar traits. Anyone with any thoughts on these 2 suggestions?

[Orange-Phantom]

What state of tune is your engine, i.e. cam head mod etc.

 

Reason why I ask is that the 7 is a colder plug and generally is used for a fast road/race spec.  (My brother uses 7's in his very highly tuned hill climber).  I've swapped from 8's to 7's in my 7 port as they were too cold.

 

If you run too cold a plug they can foul up.

 

If your car is decently tuned up then the 7's may be the correct choice.  Best to check what colour you spark plugs are at the moment and go from there.

[Dusky]

Keep the 6 plugs.
I had exactly the Same problem when I had twin hs2's on my 998 hot Mini. Dashpot oil was too thick. Solved it by changing to the proper SU oil.

[whistler]

It's a 1293 with a 'stage 4' head. 12g940 with 35.7/31mm valves and about 10.5 cr. HIF44 with bdl, 65dm4 electronic dizzy. Dashpot oil is 3 in 1 which I always use.

 

Plugs showing lean burn on centre insulator but light soot on outer rim.

Attached Files

•  20160810_151248.jpg   50.37K   12 downloads

[Dusky]

Hook it up on a gas tester at a local garage, but looks too lean to me too, pull the choke a bit when it does this agai ,should improve :)

[whistler]

Hook it up on a gas tester at a local garage, but looks too lean to me too, pull the choke a bit when it does this agai ,should improve :)

I'll give the choke a try. I'm currently looking for a Rolling Road in my area that can 'do' SU carb.

[whistler]

Booked in for RR on Thursday.

[RsSpyder]

I'm having similar trouble with mine, will be interested to hear what they say when you have it RR'd. Please keep us updated! :)

[whistler]

I'm having similar trouble with mine, will be interested to hear what they say when you have it RR'd. Please keep us updated! :)

Ok, will do.

[whistler]

Just returned from John Nutts RR session. Much improvement. He reset the timing and reconnected the vacuum advance pipe which I'd blocked off. Did a base run then had to thin the needle out as it was running far too lean up the revs. As I'm limiting the revs to 4000rpm (only done about a 1000 miles so far) he couldn't do a full power run but even at 4000 rpm it was showing good torque. Going to go back when I've done another 1000 or more miles then he can run it up to 6000+ rpm.

[liirge]

Just returned from John Nutts RR session. Much improvement. He reset the timing and reconnected the vacuum advance pipe which I'd blocked off. Did a base run then had to thin the needle out as it was running far too lean up the revs. As I'm limiting the revs to 4000rpm (only done about a 1000 miles so far) he couldn't do a full power run but even at 4000 rpm it was showing good torque. Going to go back when I've done another 1000 or more miles then he can run it up to 6000+ rpm.

So, what was the actual problem lower down the range; just the timing and the vacuum advance? Where did he plumb the VA into at the carb end, there's a specially positioned barb that is there to be used that specifically stops the advance from working when the throttle is closed. It is very important for good running to ensure that this vacuum port is used rather than one just plumbed into the manifold.

At 1000 miles it should be fine to rev at the top of the range, I have mine to 7k and it's only got about 500 miles on it. I don't want to start an argument about running in, but I do tend to veer towards the driving it as normal as you can and don't be overly cautious school of thought.

* on mobile sorry if the auto spelling has made any errors.

[whistler]

I think mainly because I had the vacuum advance from the carb blocked off. He said that if I wanted to do that then I should have either used a non vac advance dizzy or had that one modified. Reckons the timing was not retarding when slowing down as it should do and I'd set the static timing too far advanced. I followed his reasoning and it has stopped the lumpy deceleration. As mentioned before he also thinned the needle down and the acceleration is quite smooth now.

I'm over cautious about running in rpm as the first 1275 short motor I built in about 1972 burnt the rings at about 300 miles as I said to hell with it I'm doing this hill climb!

I'll give a while then go back to him.

[RsSpyder]

I think mainly because I had the vacuum advance from the carb blocked off. He said that if I wanted to do that then I should have either used a non vac advance dizzy or had that one modified. Reckons the timing was not retarding when slowing down as it should do and I'd set the static timing too far advanced. I followed his reasoning and it has stopped the lumpy deceleration. As mentioned before he also thinned the needle down and the acceleration is quite smooth now.

I'm over cautious about running in rpm as the first 1275 short motor I built in about 1972 burnt the rings at about 300 miles as I said to hell with it I'm doing this hill climb!

I'll give a while then go back to him.

 

Thanks for reporting back on this! I had suspected the issue with mine was timing - looks like i've got a job this weekend then! :)