4age with 272 cams and carbs

jakes_hachiroku
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4age with 272 cams and carbs

Postby jakes_hachiroku » Sun Oct 01, 2017 12:03 pm

So this whole summer I've been running my 4age (3 rib block bored 1mm over and a stock small port head) without issues. It also has weber 40 dcoe carbs. So the other day I though I would pep it up. Put some 272 cams with 8.1 lift. Now it still idles fine but when it gets the 3000 rpm its stumbles like crazy but once it passes it it has power again and before 3000 its fine as well. So I changed my main jets from 115 to 125 and my idle jets from 45 to 50 and every ignition timing I could try before getting upset haha any idea? Is it just my carbs not transitioning right or is it timing or both??

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frankist
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Re: 4age with 272 cams and carbs

Postby frankist » Sun Oct 01, 2017 7:05 pm

Sounds spark related. I would double check timing and make sure with a timing light that you aren't losing spark at that rpm/load.

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jondee86
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Re: 4age with 272 cams and carbs

Postby jondee86 » Sun Oct 01, 2017 10:18 pm

The usual effect of installing longer duration cams is to move peak torque/power higher
up the engines rpm range. This results in some loss of VE down low and an increase in VE
up high. I'm not familiar with the range of mixture adjustments available in the DCOE
carbs, but the engine may be over-fueling at the speed where the stumble occurs. It might
be worth trying to reduce fuel around 3000rpm.

Cheers... jondee86
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jakes_hachiroku
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Re: 4age with 272 cams and carbs

Postby jakes_hachiroku » Mon Oct 02, 2017 4:21 am

jondee86 wrote:The usual effect of installing longer duration cams is to move peak torque/power higher
up the engines rpm range. This results in some loss of VE down low and an increase in VE
up high. I'm not familiar with the range of mixture adjustments available in the DCOE
carbs, but the engine may be over-fueling at the speed where the stumble occurs. It might
be worth trying to reduce fuel around 3000rpm.

Cheers... jondee86

Hmmmm I never thought of OVER fueling but the same issue happens with 115 main jets and 125 main jets same with the idle jets. How else doyou think I could slow fuel down in that respect

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davew7
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Re: 4age with 272 cams and carbs

Postby davew7 » Mon Oct 02, 2017 5:05 am

The easiest way to tell if the mixer is rich at 3000 rpm. Does it run better when it is cold, Vs up to temp. If so, then you have a rich mixer. Dave w7

jakes_hachiroku
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Re: 4age with 272 cams and carbs

Postby jakes_hachiroku » Mon Oct 02, 2017 6:30 am

davew7 wrote:The easiest way to tell if the mixer is rich at 3000 rpm. Does it run better when it is cold, Vs up to temp. If so, then you have a rich mixer. Dave w7


it seems to run the same through out the temperature range. just started up/160/180 seems to do it at the same rpm. heres where I think its a little weird! if im starting from a stop and just floor it, first and second gear when im still not going terribly fast is fine! but when im moving it does the whole stumble at 3000 rpm again! and it only happens when im flooring it as well if Im driving and just slowly let it pas 3 then floor it im good idk maybe the choke is too small? idk lol

frankist
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Re: 4age with 272 cams and carbs

Postby frankist » Mon Oct 02, 2017 8:00 am

You sure your accelerator pump is working? sounds like a flat spot from instant flooring. Again I would double check that you have spark and that it isn't blowing out/losing spark at that rpm/load. Idle jets shouldn't have anything to do that rpm. What's your whole set up? Pictures would be cool as I'm debating whether to go DCOE or FCR.

jakes_hachiroku
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Re: 4age with 272 cams and carbs

Postby jakes_hachiroku » Mon Oct 02, 2017 8:20 am

frankist wrote:You sure your accelerator pump is working? sounds like a flat spot from instant flooring. Again I would double check that you have spark and that it isn't blowing out/losing spark at that rpm/load. Idle jets shouldn't have anything to do that rpm. What's your whole set up? Pictures would be cool as I'm debating whether to go DCOE or FCR.


so right now they are dcoe 40 carbs and the main jets are 125 and the idles are 50 f8 if I remember correctly with 30 chokes. and im using an msd 6al for ignition. and if I floor it from a stop and rip through first and second gear it doesn't stumble at all only when im traveling at regular road speeds maybe like 30mph and up. for example if im traveling at 25 or 30mph and Im in third gear say I let it go down to 2000 rpm and then floor it, it will take a little bit to get to 3 grand because that's totally the wrong gear but by the time it does the pump jets are through working already and it will have the same issure at 3000. I just cant tell am I leaning out when more air is getting to the carbs or am I too rich at that speed and rpm because its fine before 3 and after 3

jakes_hachiroku
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Re: 4age with 272 cams and carbs

Postby jakes_hachiroku » Tue Oct 03, 2017 4:16 pm

ITS FIXED!! KINDA solo I thought about the over fueling at 3000 topic that jondee brought up. So I got to thinking. What if I just matched the airflow to the fuel flow. So today after work I just simply removed the 4 chokes from the carbs, took it for a few rips and drifts around the block and had NOOOOO stumble issues and seems to be OK now!! Any future issues that might arise from this mod??

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jondee86
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Re: 4age with 272 cams and carbs

Postby jondee86 » Wed Oct 04, 2017 3:18 am

As I understand it, the chokes are used to match the carbs to the airflow of the
engine. More airflow means you use a bigger choke. Longer cams allow the engine
to suck in more air once they start working, so going to a larger choke (or removing
the chokes if you are already close to the maximum size), sounds reasonable.

The chokes are probably there to try and keep the air velocity in the carb close to
constant with different engines. I'd guess you were sucking in more air which upset
the pressure balance and caused a rich condition. Taking the chokes out dropped
the velocity in the throat and allowed the jets (emulsion tubes or whatever) to work
properly and give to a better mixture.

You really need to have a wideband O2 gauge to check that you are not too rich or
too lean. But if your plugs are some shade of tan and you don't smell fuel from the
exhaust, you should be good to go :)

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

jakes_hachiroku
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Posts: 60
Joined: Wed Jul 15, 2015 1:25 pm

Re: 4age with 272 cams and carbs

Postby jakes_hachiroku » Wed Oct 04, 2017 5:03 am

jondee86 wrote:As I understand it, the chokes are used to match the carbs to the airflow of the
engine. More airflow means you use a bigger choke. Longer cams allow the engine
to suck in more air once they start working, so going to a larger choke (or removing
the chokes if you are already close to the maximum size), sounds reasonable.

The chokes are probably there to try and keep the air velocity in the carb close to
constant with different engines. I'd guess you were sucking in more air which upset
the pressure balance and caused a rich condition. Taking the chokes out dropped
the velocity in the throat and allowed the jets (emulsion tubes or whatever) to work
properly and give to a better mixture.

You really need to have a wideband O2 gauge to check that you are not too rich or
too lean. But if your plugs are some shade of tan and you don't smell fuel from the
exhaust, you should be good to go :)

Cheers... jondee86




Thanks man! the plugs do look pretty good (tan ish color) but there is that gas smell from the exhaust not crazy bad but its there. I guess rich is safe for now?? lolol but yea I will be putting a guage on there soon probably put my 115 main jets back in to be safe but thanks for the help that's why I come to this forum only! ;)

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