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 Post subject: STR91 experience
PostPosted: Sun Oct 20, 2013 8:45 am 
Can anyone share there experience with driving with an STR91 on the road?

My Sprint is running and drivable for the first time in 3 years (yay) after a couple of days with an tuner using an 02 sensor plumbed into the primary pipes.
The bloke that has done the tuning came recommended and seems to know his stuff.

He had the following comments:
- cam may be too big for the vehicle/application*
- engine compression may be a bit low for the cam (140psi from compression test)
- may want to consider advancing cam 2 - 6 deg

My experience driving today:
- good part-throttle response
- cough/splutters under load to around 2,500rpm
- solid power delivery from 3,500rpm to ... (just wants to keep going)

* I guess the application I had in mind was fast road/track day.

Does this fit with other peoples experiences?
Is advancing the cam a few degrees a sensible option to gain some low-down drivability?


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 Post subject: Re: STR91 experience
PostPosted: Sun Oct 20, 2013 5:16 pm 
What carbs are you using? Spluttering is usually a fueling issue in my experience.


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 Post subject: Re: STR91 experience
PostPosted: Sun Oct 20, 2013 10:34 pm 
2x Weber 45 DCOE


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 Post subject: Re: STR91 experience
PostPosted: Mon Oct 21, 2013 9:22 am 
In that case it's almost guaranteed to be the idle jets if it's still hesitant when warm, what size are they?


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 Post subject: Re: STR91 experience
PostPosted: Mon Oct 21, 2013 11:23 am 
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Joined: Fri Oct 06, 2006 9:57 am
Posts: 669
Location: Auckland, New Zealand
When I first set up my car with the STR139 on 45 Dellortos i had a similar issue with hesitancy / coughing mid acceleration and that was cured by enlarging the appropriate jets (not the main jets).

I think the STR 0091 was the originally homologated rally cam so it could just be a bit wilder than you were expecting. However, I can still drive mine on the road with the STR139, just never get caught in heavy traffic as it just isn't tractible in low revs. Far worse performance than a standard car until 3000 rpm, then it increases slowly and only really flies between 5000 and 7500.

You may also be expecting too much at low revs, nothing much may happen at 2500, even when correctly tuned. The fun is all at the top end. I do find I can get more out of the bottom end by not giving too much throttle, almost feathering, waiting for the engine to pick up and only flooring the throttle after hitting the rev band.

The STR139 is timed to 102 degrees against standard of 106(?). Also has much reduced cold clearances of 7and 8 thou. Can't find any data on your cam. Can the company you bought it from supply more set up data?

When can you do your first track day? That will tell you if you have built the car you want. If so, then you may need to get used to driving to suit what the car gives you on the road. More gear changes and keeping the revs up.

Geoff


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 Post subject: Re: STR91 experience
PostPosted: Mon Oct 21, 2013 11:36 am 
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Joined: Thu Oct 06, 2011 1:13 am
Posts: 3173
Location: The continent
The str91 is a good usable cam for road use. Power starts very early and very good driveability. Specially designed for rally so torque at low revs.

This one has also a str91 and has iirc from 2500-5500 at least 75% of max torque available.

A healthy std engine with str91 and webers should have around 160-170bhp.


http://sprint.smugmug.com/Cars/Forum/11 ... &k=XFgtVmS

Jeroen

_________________
Classic Kabelboom Company. For all your wiring needs. http://www.classickabelboomcompany.com


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 Post subject: Re: STR91 experience
PostPosted: Mon Oct 21, 2013 11:51 pm 
If you're on idle jets of .55 or less they're too small. A Sprint should run fine on a 60F8, a great starting point.


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 Post subject: Re: STR91 experience
PostPosted: Tue Oct 22, 2013 10:54 pm 
A number of problems could be apparent and maybe the guy who set it upwas not as good as you thought, Spluttering is ususlly a fuelling issue, some possible causes are; incorrect idle jets, incorrect emulsion tubes, too large chokes. I have run a 139 on the road and the car is perfectly useable and tractable even down at 1,000 rev/min with careful use of the throttle.

Putting a 'wild' cam in an otherwise standard engine is not usually a good idea, by increasing the valve overlap you effectively decrease the compression ratio so any supposed gain is negated by the lower CR.


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 Post subject: Re: STR91 experience
PostPosted: Tue Oct 22, 2013 11:49 pm 
Thanks all,

I'll check my receipt re jetting and post an update tonight


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 Post subject: Re: STR91 experience
PostPosted: Wed Oct 23, 2013 9:42 am 
Ok, so here's some detail:

- 135 main jets
- 180 aux jets
- 60F8 idle jets

New spark plugs
The engine idles at 1000-1200rpm

Cam data
- Timing: 40/73-68/32
- Lift: 0.383"
- Duration: 286 °
- Power band: 3000-7000
- Est power: 178 bhp
- Max lift: 106.5 °
- Clearance: In: 10/11 thou, Ex: 12/13 thou

Other possibly relevant stuff is:
- large bore primary pipes
- 2" sports exhaust
- digital multispark ignition
- vernier cam gear (should I want to go advancing the cam)


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 Post subject: Re: STR91 experience
PostPosted: Fri Oct 25, 2013 12:41 am 
I'm still thinking fueling. Make sure the progression channels, holes and jets are all clear.
At roughly 2500 the carbs switch onto the main jet stack so it must be something to do with the progression circuit.


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 Post subject: Re: STR91 experience
PostPosted: Fri Oct 25, 2013 9:18 am 
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Joined: Thu Oct 06, 2011 1:13 am
Posts: 3173
Location: The continent
First make sure the ignition timing is about 10-15 degrees at idle. Multi carbs with a tuned cam have a not so good idle mixture so the ignition must start earlier.

Rule of thumb:

Your ideal idle mixture is when the engine does rev the highest. This sequence one cilinder at the time. At idle screw the idle screw in till the rev drops. Then start turning the screw out again till the revs are at highest. When it start dropping again you are too far and the mixture is to rich. So turn in till drop, then out again till highest. Repeat with all cilinders. It is normal that not all the screws are turned out equally but within limits. If the idle is too high now you have to adjust the throttle valves to normal idle or if you have newer webers there's an extra idle air bore to have each cilinder fine balanced but at the trottle valve should be fine for now.

If the screws are adjusted and you have normal idle they must be about 1,5-2,5 turns out. If this is your idle/partload jets are fine and the car should run without major problems. if the screws are turned in more the jets are too big. If the screws are turned out more than 2,5 turns the jets are too small.

You also can check if all cilinders do the same. When closing the screws one by one, that cilinder does nothing anymore. If by turning in a cilinder does not react, or react less there's something wrong.

Another thing to look at is with a mirror in the carbs when idling to check if the accelerator jets are not dripping. This way the idle mixture is wrong adjusted. It enrichens the mixture. If they drip and your screws are adjusted within limits the jets are still to small because at idle you add the dripping fuel to your mixture and think all is ok but when driving there's a short of fuel because of too small jets.

This way you can drive your engine safely but it is not most ideal. For the best results and economy it's only possible on a rolling road.

Jeroen

_________________
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