Fast Idle on Sprint

[samgamgee911]

Hi, could you please let me know you thoughts on this. Car starts OK and idles back when warm, nicely to about 800 rpm. Take the car for a drive, stop at road junction and the car is running at 1500 rpm. Back home car running high in garage about 1200 rpm but after some time idle reduces back to 800 rpm. Could this be a sticky linkage somewhere, one that is slow to return to home position?<br>
<br>
Thanks in advance<br>
Stephen

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[alun n]

Check dash pot oil; if its low a blip on the throttle will return the speed to proper idle when its racing. Also look at the position of the clamps on the linkeage, one may be holding a carb open when other is shut. Could also be an air leak...

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[samgamgee911]

Hi, the dash pot oil was a little low, not much and the linkages seem fine, I put some light oil on them anyway but it hasn't improved the situation. How do u check for leaks? I treid spraying WD40 on rubber mounts but it hasn't changed - what else can I try in relation to looking for leaks?<br>
<br>
Thanks again in advance<br>
Stephen<br>


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[davepoth]

Basically spray wd40 anywhere on the carbs where there is a joint after the metering needles, i.e. carb body, rubber mounts, and where the manifold meets the engine. if the engine speed goes up when you spray at any of them, you've got an air leak which will probably be the cause of the rising idle. <br>
<br>
Apart from the high idle, are there any other problems with running?

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[LewisK]

It's funny, mine suffers EXACTLY the same symptom. Not had a chance to poke it yet but it is quite odd!

<p><!--EZCODE IMAGE START--><img src="http://www.antchip.com/sig2.jpg"/><!--EZCODE IMAGE END--><br>
<!--EZCODE FONT START--><span style="font-size:xx-small;"><!--EZCODE ITALIC START--><em>Crazy! I mean like so many positive waves, we can't lose! You're on!</em><!--EZCODE ITALIC END--></span><!--EZCODE FONT END--></p><i></i>

[samgamgee911]

Hi - no other problems with running, if fact it is going the best ever in the 2 years I've owned it ( probably because of the amount of new parts put on it mind you) and that was why I was aksing before going poking around myself as I know nothing about carbs and how to set them up.<br>
<br>
Let me know Lewis if you come to the root cause.<br>
Cheers

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[2F45T4U]

sticking? I think Richards got troubles with his. It takes about 10 seconds to fully idle down!

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[no41sprint]

my does too ,now found the cause is split carb mounts.<br>
have slowed the carbs down with the adjuster nut as was getting very warm.have i read somewhere that the club do the carb mounts?

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[Jon Tilson]

are several possible causes for this..<br>
<br>
1 Crud around linkages<br>
2 Sticking throttle cable or linkage<br>
3 Sticking choke linkage<br>
4 Bad carb adjustment<br>
5 Bad ignition timing<br>
6 Air leaks<br>
7 Worn carbs<br>
8 Weak return springs<br>
8 anything else.<br>
<br>
Poppet valves on throttle discs can be the cause. Usually dirt causes them not to seat and let to much air in. Often blipping the throttle will cure this.<br>
<br>
How to diagnose....<br>
<br>
Visual check on all rubber bits, breather pipes etc. Anything suspicious should be replaced. Then remove air filter and loosen the throttle fork clamps so the carbs can be set independant of the throttle linkage. Make sure the choke returns full home and the bits that move the jets up and down are free and dont bind. Then set the idle speed as you would like it with the screwdriver on the idle screw. If reducing the speed as much as possible doesnt bring it down enough you have a leaking spindle or stuck poppet valve.<br>
You can balance the carbs with the old rubber pipe in the ear as a stethoscope trick. Listen to the hiss and set them the same.<br>
Then set the fast idle cam screws that up the idle a bit when you pull the choke a tad. You dont want more than 1500 rpm from these.<br>
Reset the linkage by retightening the clamp screws and make sure it all returns freely. You want a bit of clearance so the idle scerws are controlling the low speed and not the linkage. Its all in the book...<br>
<br>
There is also that irrirtating throttle progression gubbins that wears as well. Check that and you will find loads of slop and wear unless you are very lucky. The only solution is to recon it with drills bolts etc. Or find a better one on a TR7...which I think are the same but dont quote me...Alun?<br>
<br>
Jonners<br>
<br>
<br>
<br>
<br>


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[DailySprinter]

I'd check these first.<br>
When the car does the slow idle, lift the carbs and air box upwards, if the revs rise they are knackered, air is getting in<br>
<br>
Only get alloy carb mounts, rubber ones only last 5 minutes

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[Jon Tilson]

The OE rubber mounts are fine on SPrints and 7's.<br>
I find my original 25+ year old ones are still air tight and prvide that essential bit of thermal isolation from the manifold and probably a bit of vibration damping too.<br>
<br>
I think there are some iffy quality new rubber ones about though.<br>
<br>
On 1850's the mountings suffer a bit more and dont last as long.<br>
<br>
Moving the airbox can identify the problem but sometimes you are actually moving the throttle as you lift the filter so its not foolproof. Carb spray with a fine tube is the only sure way of identfying the air leak.<br>
Jonners

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[redrichie]

I had this problem last summer, it was the poppet valves sticking open. There are three solutions; solder the poppet valves shut, change them for plain discs, or fit new discs with new poppet valves.<br>
<br>
I went for the first option. <!--EZCODE EMOTICON START ;) --><img src=http://www.ezboard.com/images/emoticons/wink.gif ALT=";)"><!--EZCODE EMOTICON END-->

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[samgamgee911]

Hi, is the poppet valve part of the brass cylinder that opens and closes on the spindle? I take it I should strip down carb to do this - how can you tell if it is faulty before hand?<br>
<br>
Thanks

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[Jon Tilson]

although I'd call it a brass disc.<br>
Its held to the throttle spindle with 2 small screws.<br>
Sprints dont usually suffer form spindle wear quite so badly as older 1850's because the HS6 carbs all have that hair spring to return the throttle rather than that external pull spring which aslo drags the spindle sideways. Well all the Sprints and 7's Ive seen are like this...maybe older ones have the inferior setup too.<br>
Best bet is to just get some plain discs...<br>
<br>
Jonners

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[redrichie]

well, a picture paints a thousand words....<br>
<br>
First off, an unsoldered poppet valve...<br>
<br>
<!--EZCODE IMAGE START--><img src="http://img54.imageshack.us/img54/3918/u ... <!--EZCODE IMAGE END--><br>
<br>
<br>
And now one that has been soldered to prevent it sticking open and allowing air to rush past (thus increasing the engine rpm)<br>
<br>
<!--EZCODE IMAGE START--><img src="http://img60.imageshack.us/img60/6455/s ... <!--EZCODE IMAGE END--><br>
<br>
<br>
Hope this helps..

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