SU HIF Carbs Set up..

haystax · Post by **haystax** » Sat May 19, 2012 9:55 am

Hi Again,I have finaly got my 3.5 V8 Running now..It starts first time ticks over well...Problem..When I first took trike for a run,It was gutless...Wouldnt pick up..When I pulled choke on it took off...The carbs have been balanced,and synched...I richened it up a bit after first run out,It was a bit better,but started to backfire thruogh carb on right bank..But again with choke out,it ran ok...Took out plugs,and were all still black as Newgates Knocker..Any Help please,I have looked at similar posts on here,but dont really see my problem ....And have Synched and balanced carbs over and over again...Still the same...

Quagmire · Post by **Quagmire** » Sat May 19, 2012 5:13 pm

Here is a copy of the manual for pretty much every type of carb ever.

http://www.sendspace.com/file/vcx9uq

Sorry about the crappy host, but with hotfile and fileserve gone its harder to put stuff up without registering.

Click the link on the page in the blue box that says "Click here to start download from sendspace" -

Don't click the big "Click here to download" as it asks you to install some random executable file (ok for me as I am on Linux, but god knows what it'll do to a windoze machine).

SU HIF's are covered on page 235 onwards I think.

ChrisJC · Post by **ChrisJC** » Sun May 20, 2012 8:48 am

Worth checking that the seals on the throttle spindle are OK.
Also worth checking that the float assembly is working properly and not flooding it.
And that the piston moves freely up and down.

Chris.

kiwicar · Post by **kiwicar** » Mon May 21, 2012 10:22 am

Hi
as said above start checking for air leaks, a tin of easy start or WD40 will help this process but firstly get all the air going in through the throttles. again as above check the float levels and float condition, especially if it has been back firing, check the float valves are sealing. Check the condition of the jets and needles, make sure they are not excessivly worn. Next re ballance the carbs, set the jet levels to a starting point that is the same for both and use the lifting pin method to get the idle set. With the air filters off have a look down into the carbs with the engine running at fast idle (800 revs ish) where are the pistons, same level? if not check the springs that control the pistons lift, do the same at 1500 revs. Do the pistons lift sit steadily in the venturies, or do they wobble about and rattle from side to side? if the latter then they are done and you need to find some newer carbs, after a point you get so much wear that you can never get them to ballance as most of the air goes around the piston and not past the jet (it is pretty obvious if they are that done though).
Now open the throttles with the engine running and watch the pistons rise as the revs go up, you want the pistons to reach the top of their lift about 500 revs before your maximum revs, so for a basic tue up engine it should be about 5300 to 5500 revs, they both need to get their the same time/revs, if not again check the springs and renew, often people compensate for a tired spring at low revs by pulling it so it is longer and gives more pre load, that does not work at higher revs and the carbs are not ballanced through the rev range.
If you haven't done it already solder the poppet valves on the throttle plates shut, you won't get the carbs ballanced and set up properly with those opening at random.
I actually think you will have this sorted by you have eliminated all the usual airleaks from things like split crank case evacuation plumbing and set up the idle, but if not the later stuff covers most of the rest.
Best regards
Mike

badger · Post by **badger** » Mon May 21, 2012 7:24 pm

Hate to say it, but having to pull the choke out with SU's is a classic symptom of a knackered camshaft.....

ChrisJC · Post by **ChrisJC** » Mon May 21, 2012 8:07 pm

badger wrote:Hate to say it, but having to pull the choke out with SU's is a classic symptom of a knackered camshaft.....

That's interesting - why is that?

Chris.

kiwicar · Post by **kiwicar** » Tue May 22, 2012 9:14 am

Hi Badger
Yup got my attention, I always associated it with air leaks and kn@ckered carb pistons I would be very interested to know more.
Best regrads
Mike

DaveEFI · Post by **DaveEFI** » Tue May 22, 2012 12:05 pm

Last carb Rover I had was a 3500S P6, and the cam on that was almost round.

Can't say I noticed the mixture had gone weak - the only major change with the new one was a much better top end - and rougher idle.

unstable load · Post by **unstable load** » Wed May 23, 2012 6:34 am

ChrisJC wrote:
badger wrote:Hate to say it, but having to pull the choke out with SU's is a classic symptom of a knackered camshaft.....
That's interesting - why is that?

Chris.

Indeed, how do you figure that one out?
I fitted a new cam to my P6 a few years back and it needed the choke right off the bat to start it and still does to this day.

badger · Post by **badger** » Wed May 23, 2012 7:57 pm

As far as I can work out, it's all to do with vacuum signals changing at the carb due to inlet and exhaust valves not opening correctly. I've seen it on so many old 3.5 rangerovers over the last 2 decades now - flat, down-on-power engine, pull choke partially out at 3000 rpm+ and there is a noticeable increase in power. Every one, when stripped, has had a completely knackered cam, mostly the exhaust lobes.
If, say, one cylinder out of the 4 that a carb feeds is "pumping" (restricted exhaust opening), then that has an effect on the vacuum pulled by that cylinder, which has a knock-on effect to the other 3 cylinders being fed by that carb as there is an overall airflow and vacuum change within the manifold. This ends up causing the piston and needle assembly to be in the wrong position, lower than it ought to be, and reducing fuel flow overall due to the taper of the needle. Airflow is fine, for 3 and a bit cylinders, but the actual air/fuel ratio has altered from where the three good cylinders need it to be.
Hope that explains it.....

kiwicar · Post by **kiwicar** » Thu May 24, 2012 9:24 am

Hi
That makes good sense to me, probably a well contaminated inlet charge to go with it, not helping.
You now have me thinking about the effects of long duration exhaust lobes on cams used with Rhoads style lifters

Best regards
Mike

unstable load · Post by **unstable load** » Sat May 26, 2012 6:55 am

Interesting idea! Thanks for that, I guess I need to look into my
cam's state of health, then.

haystax · Post by **haystax** » Thu Jun 07, 2012 3:47 pm

Thanks all for your help..Got some more carbs to try first,to eliminate that side of the problem.Will keep you informed...

DaveEFI · Post by **DaveEFI** » Thu Jun 07, 2012 4:56 pm

haystax wrote:Thanks all for your help..Got some more carbs to try first,to eliminate that side of the problem.Will keep you informed...

SU carbs are pretty simple devices. Changing them for another unknown pair may not be the best way. Better to strip them and examine for wear etc - although they have to be very badly worn to cause severe problems. The most usual ones are easily fixed.

haystax · Post by **haystax** » Fri Jun 08, 2012 9:43 am

I find it strange.It was running ok with them before i blew head gasket..Since rebuild,it has been crap..I have colortuned it now,and got electric blue burn,but cant take it for test run coz of bloody rain...Have cleaned them new seals etc...When rain stops,will test it again..