SU Carbie issue

[Austin 1800]

Hi Everyone,

I have an Austin 1800 Mk1 automatic, the car is running great in that it idles fine, starts fine and does not stall.. ever.... the issue I have is that under load is just seems to be bogging down a bit, it also struggles a bit going up hills... cruising along at 80kms per hr it is perfect... sitting on 100kms on the expressway it seems to be making a strange sucking sound and bogs down slightly when I put the foot down to overtake etc....

It has new oil, new spark-plugs, a Scorcher electronic ignition   (https://performanceignition.com.au/p/scorcher)... new air filter and oil filters... I do also run flashlube (https://www.flashlube.com/en/products/valve-saver-kit.html) even though I do have hardened valve seats... I run the car on 95 octane fuel and sometimes 98... it is a HS6 carb (1.75")

If anyone has any thoughts on why it would bog down a bit at higher revs or under load then I would greatly appreciate your assistance... I though maybe vacuum advance was not working correctly? but I think that would make it more lean than anything wouldn't it??

Thanks.

Austin guy.

[240260280]

When you transition, it is the damper that prevents the piston (and needle) from moving too fast or too slow. @Austin 1800

 

If the damper opens too fast, you will go lean during transition, if it opens too slow, you will go rich during transition. You can experiment by removing the oil from the plunger and test driving then replace the oil with a thick 30wt and test driving to see the two extreme transition speeds of the piston and judge if this is helpful.

 

As you mention it could be advance.  It also could be SU piston binding or wrong needle profile.  Are your carbs balanced?  If not, have a look here: https://www.classiczcars.com/forums/topic/39221-quick-and-dirty-su-tuning/?tab=comments#comment-352697

 

Edited June 14, 2018 by 240260280

[240260280]

Also check all 4  plugs at idle for baseline colour then run at highway speed for 5min then do an ignition cut and coast to the side of the road  to check plugs.  This will tell you if you are running richer or leaner than idle at highway speed.  If your carbs are balanced, valves are sealing, no intake leaks, and ignition is good then the difference is due to your needle shape.

Edited June 14, 2018 by 240260280

[siteunseen]

My choice and the best for my climate is...

[Austin 1800]

Thanks very much for your comments... I only have a single carbie....

 

As an experiment I pulled the choke out a little when on the expressway home last night... it made a big difference, the car seems to free up, going up hills was easier too... I have no doubt now that it is running too lean at higher revs.... I should also say that the recent work I did on the car was as follows: New oil, plugs and filters, and I also re-adjusted the rockers as the were quite loose (Except one which was too tight)... all the rattle sound has now gone and it sounds fantastic... I should also say that it has a "Too stages above standard" cam in it what ever that means...  that it what the guy said when it was rebuilt back in 2001. Since then it did I would say 20 thousand kms then sat the then shed for 5 years due to a broken part.... then I got it back on the road again last August... since then it has done about 15 thousand kms as I use it to and from work and have also driven up to Sydney a few times (2000kms round trip).

Thanks  ?

[240260280]

Lower the jet.  Please post a photo of your intake and carb.

[240260280]

Lower mixture adjusting nut to enrich the engine.

 

[zKars]

You're just running lean. If this is a new condition, I'd suspect your fuel supply has been comprimised. Dirty filter, plugged lines, poorly performing fuel pump.  On the other side of the coin, a vacuum leak lets more air in that is expected leading to a lean mixture even if the fuel system is working well.